Error connecting to MSSQL

Issue:
Error when trying to connect to MSSQL database:
```
Warning: QSqlDatabase: QODBC driver not loaded
Warning: QSqlDatabase: available drivers: QSQLITE
```

Steps to Reproduce:
1. MSSQL > New Connection ..
2. Filled in `Host`
3. Unchecked "Trusted connection"
4. Entered a username and password
5. Clicked `List Databases` and `Test Connection`, both result in error.

I tested the same host/creds using pymssql (freetds) from the QGIS python console and was able to connect and make queries.

Environment (conda list):

```
# packages in environment at /home/ubuntu/miniconda3/envs/qgis-env:
#
# Name Version Build Channel
asn1crypto 0.24.0 py37_1003 conda-forge
atk 2.25.90 hb9dd440_1002 conda-forge
attrs 19.1.0 py_0 conda-forge
boost-cpp 1.68.0 h11c811c_1000 conda-forge
bzip2 1.0.6 h14c3975_1002 conda-forge
ca-certificates 2019.3.9 hecc5488_0 conda-forge
cairo 1.16.0 ha4e643d_1000 conda-forge
certifi 2019.3.9 py37_0 conda-forge
cffi 1.12.3 py37h8022711_0 conda-forge
cfitsio 3.430 h0bd2449_1002 conda-forge
chardet 3.0.4 py37_1003 conda-forge
coverage 4.5.3 py37h14c3975_0 conda-forge
cryptography 2.7 py37h72c5cf5_0 conda-forge
curl 7.64.1 hf8cf82a_0 conda-forge
dbus 1.13.6 he372182_0 conda-forge
decorator 4.4.0 py_0 conda-forge
exiv2 0.27.1 had08079_0 conda-forge
expat 2.2.5 hf484d3e_1002 conda-forge
fontconfig 2.13.1 he4413a7_1000 conda-forge
freetds 1.1rc3 h4fe99da_0 conda-forge
freetype 2.10.0 he983fc9_0 conda-forge
freexl 1.0.5 h14c3975_1002 conda-forge
future 0.17.1 py37_1000 conda-forge
gdal 2.4.1 py37h5f563d9_4 conda-forge
gdk-pixbuf 2.36.12 h49783d7_1002 conda-forge
geos 3.7.1 hf484d3e_1000 conda-forge
geotiff 1.5.1 h560c3f3_2 conda-forge
gettext 0.19.8.1 hc5be6a0_1002 conda-forge
giflib 5.1.7 h516909a_1 conda-forge
glib 2.58.3 hf63aee3_1001 conda-forge
gobject-introspection 1.58.2 py37h2da5eee_1000 conda-forge
graphite2 1.3.13 hf484d3e_1000 conda-forge
gsl 2.4 h294904e_1006 conda-forge
gst-plugins-base 1.14.5 h0935bb2_0 conda-forge
gstreamer 1.14.5 h36ae1b5_0 conda-forge
gtk2 2.24.31 hb68c50a_1001 conda-forge
harfbuzz 2.4.0 h37c48d4_0 conda-forge
hdf4 4.2.13 h9a582f1_1002 conda-forge
hdf5 1.10.5 nompi_h3c11f04_1100 conda-forge
httplib2 0.13.0 py37_0 conda-forge
icu 58.2 hf484d3e_1000 conda-forge
idna 2.8 py37_1000 conda-forge
ipython_genutils 0.2.0 py_1 conda-forge
jinja2 2.10.1 py_0 conda-forge
jpeg 9c h14c3975_1001 conda-forge
json-c 0.13.1 h14c3975_1001 conda-forge
jsonschema 3.0.1 py37_0 conda-forge
jupyter_core 4.4.0 py_0 conda-forge
kealib 1.4.10 h58c409b_1005 conda-forge
krb5 1.16.3 h05b26f9_1001 conda-forge
libblas 3.8.0 7_openblas conda-forge
libcblas 3.8.0 7_openblas conda-forge
libcurl 7.64.1 hda55be3_0 conda-forge
libdap4 3.19.1 hd48c02d_1000 conda-forge
libedit 3.1.20170329 hf8c457e_1001 conda-forge
libffi 3.2.1 he1b5a44_1006 conda-forge
libgcc-ng 9.1.0 hdf63c60_0
libgdal 2.4.1 h654d158_4 conda-forge
libgfortran-ng 7.3.0 hdf63c60_0
libiconv 1.15 h516909a_1005 conda-forge
libkml 1.3.0 h328b03d_1009 conda-forge
liblapack 3.8.0 7_openblas conda-forge
libnetcdf 4.6.2 h056eaf5_1002 conda-forge
libpng 1.6.37 hed695b0_0 conda-forge
libpq 11.3 h4e4e079_0 conda-forge
libspatialindex 1.9.0 he1b5a44_1 conda-forge
libspatialite 4.3.0a h4c5cd43_1028 conda-forge
libssh2 1.8.2 h22169c7_2 conda-forge
libstdcxx-ng 9.1.0 hdf63c60_0
libtiff 4.0.10 h57b8799_1003 conda-forge
libuuid 2.32.1 h14c3975_1000 conda-forge
libxcb 1.13 h14c3975_1002 conda-forge
libxml2 2.9.9 h13577e0_0 conda-forge
libxslt 1.1.32 h4785a14_1002 conda-forge
libzip 1.5.1 h1a52d62_1005 conda-forge
lxml 4.3.4 py37h7ec2d77_0 conda-forge
lz4-c 1.8.3 he1b5a44_1001 conda-forge
markupsafe 1.1.1 py37h14c3975_0 conda-forge
mock 3.0.5 py37_0 conda-forge
nbformat 4.4.0 py_1 conda-forge
ncurses 6.1 hf484d3e_1002 conda-forge
nose2 0.9.1 py_0 conda-forge
numpy 1.16.4 py37h95a1406_0 conda-forge
openblas 0.3.5 h9ac9557_1001 conda-forge
openjpeg 2.3.1 h58a6597_0 conda-forge
openssl 1.1.1b h14c3975_1 conda-forge
owslib 0.17.1 py_0 conda-forge
pango 1.40.14 h4ea9474_1004 conda-forge
pcre 8.41 hf484d3e_1003 conda-forge
pip 19.1.1 py37_0 conda-forge
pixman 0.34.0 h14c3975_1003 conda-forge
plotly 3.10.0 py_0 conda-forge
poppler 0.67.0 h2ad7f00_6 conda-forge
poppler-data 0.4.9 1 conda-forge
postgresql 11.3 hbeee2d4_0 conda-forge
proj4 6.1.0 he751ad9_2 conda-forge
psycopg2 2.8.2 py37h72c5cf5_0 conda-forge
pthread-stubs 0.4 h14c3975_1001 conda-forge
pycparser 2.19 py37_1 conda-forge
pygments 2.4.2 py_0 conda-forge
pymssql 2.1.4 py37hf484d3e_2001 conda-forge
pyopenssl 19.0.0 py37_0 conda-forge
pyproj 2.2.0 py37hc44880f_0 conda-forge
pyqt 5.9.2 py37hcca6a23_0 conda-forge
pyrsistent 0.15.2 py37h516909a_0 conda-forge
pysocks 1.7.0 py37_0 conda-forge
python 3.7.3 h5b0a415_0 conda-forge
python-dateutil 2.8.0 py_0 conda-forge
pytz 2019.1 py_0 conda-forge
pyyaml 5.1.1 py37h516909a_0 conda-forge
qca 2.2.1 h279c94e_1 conda-forge
qgis 3.6.3 py37h758d154_2 conda-forge
qjson 0.9.0 h279c94e_1005 conda-forge
qscintilla2 2.11.1 py37hd2e5223_2 conda-forge
qt 5.9.7 h52cfd70_2 conda-forge
qtkeychain 0.9.1 h279c94e_1001 conda-forge
qtlocation 5.9.7 he1b5a44_0 conda-forge
qtserialport 5.9.8 h279c94e_0 conda-forge
qwt 6.1.4 h279c94e_1003 conda-forge
qwtpolar 1.1.1 h279c94e_6 conda-forge
readline 7.0 hf8c457e_1001 conda-forge
requests 2.22.0 py37_0 conda-forge
retrying 1.3.3 py_2 conda-forge
setuptools 41.0.1 py37_0 conda-forge
sip 4.19.8 py37hf484d3e_1000 conda-forge
six 1.12.0 py37_1000 conda-forge
sqlite 3.28.0 h8b20d00_0 conda-forge
tk 8.6.9 hed695b0_1002 conda-forge
traitlets 4.3.2 py37_1000 conda-forge
tzcode 2019a h516909a_1002 conda-forge
unixodbc 2.3.7 h227dcee_1000 conda-forge
urllib3 1.24.3 py37_0 conda-forge
wheel 0.33.4 py37_0 conda-forge
xerces-c 3.2.2 hac72e42_1001 conda-forge
xorg-inputproto 2.3.2 h14c3975_1002 conda-forge
xorg-kbproto 1.0.7 h14c3975_1002 conda-forge
xorg-libice 1.0.9 h516909a_1004 conda-forge
xorg-libsm 1.2.3 h84519dc_1000 conda-forge
xorg-libx11 1.6.7 h14c3975_1000 conda-forge
xorg-libxau 1.0.9 h14c3975_0 conda-forge
xorg-libxdmcp 1.1.3 h516909a_0 conda-forge
xorg-libxext 1.3.4 h516909a_0 conda-forge
xorg-libxrender 0.9.10 h516909a_1002 conda-forge
xorg-libxt 1.1.5 h14c3975_1002 conda-forge
xorg-renderproto 0.11.1 h14c3975_1002 conda-forge
xorg-xextproto 7.3.0 h14c3975_1002 conda-forge
xorg-xproto 7.0.31 h14c3975_1007 conda-forge
xz 5.2.4 h14c3975_1001 conda-forge
yaml 0.1.7 h14c3975_1001 conda-forge
zlib 1.2.11 h14c3975_1004 conda-forge
zstd 1.4.0 h3b9ef0a_0 conda-forge
```

Details about conda and system ( conda info ):

```

active environment : qgis-env
active env location : /home/ubuntu/miniconda3/envs/qgis-env
shell level : 2
user config file : /home/ubuntu/.condarc
populated config files :
conda version : 4.6.14
conda-build version : not installed
python version : 3.7.1.final.0
base environment : /home/ubuntu/miniconda3 (writable)
channel URLs : https://repo.anaconda.com/pkgs/main/linux-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/free/linux-64
https://repo.anaconda.com/pkgs/free/noarch
https://repo.anaconda.com/pkgs/r/linux-64
https://repo.anaconda.com/pkgs/r/noarch
package cache : /home/ubuntu/miniconda3/pkgs
/home/ubuntu/.conda/pkgs
envs directories : /home/ubuntu/miniconda3/envs
/home/ubuntu/.conda/envs
platform : linux-64
user-agent : conda/4.6.14 requests/2.19.1 CPython/3.7.1 Linux/4.15.0-50-generic ubuntu/18.04.2 glibc/2.27
UID:GID : 1000:1000
netrc file : None
offline mode : False
```