Can't rerender feedstock

- [x] I read [the conda-forge documentation](https://conda-forge.org/docs/user/introduction.html#how-can-i-install-packages-from-conda-forge) and could not find the solution for my problem there.

Issue:
Running `conda smithy rerender` locally or on a PR fails with error:
```
ValueError: variant config in /Users/freddy.boulton/sources/personal/python-kaleido-feedstock/recipe/conda_build_config.yaml is ambiguous because it does not fully implement all zipped keys, or specifies a subspace that is not fully implemented.
```

This happens in this feedstock and in the (evalml-core feedstock)[https://github.com/conda-forge/evalml-core-feedstock], which uses the same `conda_build_config.yaml` file to specify a centos 7 docer image.

I'd like us to figure out the changes we need to make to rerender our feedstocks!

Environment (conda list):

```
$ conda list
beautifulsoup4 4.9.3 pyhb0f4dca_0 conda-forge
blinker 1.4 py_1 conda-forge
boolean.py 3.7 py_0 conda-forge
brotlipy 0.7.0 py38h94c058a_1001 conda-forge
bzip2 1.0.8 hc929b4f_4 conda-forge
c-ares 1.17.1 hc929b4f_0 conda-forge
ca-certificates 2020.12.5 h033912b_0 conda-forge
certifi 2020.12.5 py38h50d1736_0 conda-forge
cffi 1.14.4 py38h979bc6b_1 conda-forge
chardet 4.0.0 py38h50d1736_0 conda-forge
conda 4.9.2 py38h50d1736_0 conda-forge
conda-build 3.20.5 py38h5347e94_0 conda-forge
conda-forge-pinning 2021.01.03.16.11.28 hd8ed1ab_0 conda-forge
conda-package-handling 1.7.2 py38h94c058a_0 conda-forge
conda-smithy 3.8.6 pyhd8ed1ab_0 conda-forge
cryptography 3.3.1 py38h6b4ec92_0 conda-forge
curl 7.71.1 hcb81553_8 conda-forge
deprecated 1.2.10 pyh9f0ad1d_0 conda-forge
expat 2.2.9 hb1e8313_2 conda-forge
filelock 3.0.12 pyh9f0ad1d_0 conda-forge
gettext 0.19.8.1 h7937167_1005 conda-forge
git 2.30.0 pl5320hd05cdc2_0 conda-forge
gitdb 4.0.5 py_0 conda-forge
gitpython 3.1.11 py_0 conda-forge
glob2 0.7 py_0 conda-forge
icu 68.1 h74dc148_0 conda-forge
idna 2.10 pyh9f0ad1d_0 conda-forge
isodate 0.6.0 py_1 conda-forge
jinja2 2.11.2 pyh9f0ad1d_0 conda-forge
krb5 1.17.2 h60d9502_0 conda-forge
libarchive 3.5.1 h0a5793d_1 conda-forge
libcurl 7.71.1 h9bf37e3_8 conda-forge
libcxx 11.0.0 h4c3b8ed_1 conda-forge
libedit 3.1.20191231 h0678c8f_2 conda-forge
libev 4.33 haf1e3a3_1 conda-forge
libffi 3.3 h046ec9c_2 conda-forge
libiconv 1.16 haf1e3a3_0 conda-forge
liblief 0.10.1 hb1e8313_2 conda-forge
libnghttp2 1.41.0 h7580e61_2 conda-forge
libssh2 1.9.0 h8a08a2b_5 conda-forge
libxml2 2.9.10 h802ca4f_3 conda-forge
license-expression 1.2 py_0 conda-forge
lz4-c 1.9.3 h046ec9c_0 conda-forge
lzo 2.10 haf1e3a3_1000 conda-forge
markupsafe 1.1.1 py38h94c058a_2 conda-forge
msrest 0.6.19 pyh9f0ad1d_0 conda-forge
ncurses 6.2 h2e338ed_4 conda-forge
oauthlib 3.0.1 py_0 conda-forge
openssl 1.1.1i h35c211d_0 conda-forge
pcre 8.44 hb1e8313_0 conda-forge
perl 5.32.0 hbcb3906_0 conda-forge
pip 20.3.3 pyhd8ed1ab_0 conda-forge
pkginfo 1.6.1 pyh9f0ad1d_0 conda-forge
psutil 5.8.0 py38h5406a74_0 conda-forge
py-lief 0.10.1 py38heedec30_2 conda-forge
pycosat 0.6.3 py38h94c058a_1005 conda-forge
pycparser 2.20 pyh9f0ad1d_2 conda-forge
pycrypto 2.6.1 py38h94c058a_1005 conda-forge
pygithub 1.44 py38h32f6830_0 conda-forge
pyjwt 1.7.1 py_0 conda-forge
pyopenssl 20.0.1 pyhd8ed1ab_0 conda-forge
pysocks 1.7.1 py38h5347e94_2 conda-forge
python 3.8.6 h624753d_4_cpython conda-forge
python-libarchive-c 2.9 py38h5347e94_2 conda-forge
python_abi 3.8 1_cp38 conda-forge
pytz 2020.5 pyhd8ed1ab_0 conda-forge
pyyaml 5.3.1 py38h94c058a_1 conda-forge
readline 8.0 h0678c8f_2 conda-forge
requests 2.25.1 pyhd3deb0d_0 conda-forge
requests-oauthlib 1.3.0 pyh9f0ad1d_0 conda-forge
ripgrep 12.1.1 haf1e3a3_1 conda-forge
ruamel.yaml 0.16.12 py38h1130bcf_1 conda-forge
ruamel.yaml.clib 0.2.2 py38h1130bcf_1 conda-forge
ruamel_yaml 0.15.80 py38h94c058a_1003 conda-forge
scrypt 0.8.17 py38h6b4ec92_1 conda-forge
setuptools 49.6.0 py38h5347e94_2 conda-forge
six 1.15.0 pyh9f0ad1d_0 conda-forge
smmap 3.0.4 pyh9f0ad1d_0 conda-forge
soupsieve 2.0.1 py_1 conda-forge
sqlite 3.34.0 h17101e1_0 conda-forge
tk 8.6.10 h0419947_1 conda-forge
toolz 0.11.1 py_0 conda-forge
tqdm 4.55.1 pyhd8ed1ab_0 conda-forge
urllib3 1.26.2 pyhd8ed1ab_0 conda-forge
vsts-python-api 0.1.22 py_0 conda-forge
wheel 0.36.2 pyhd3deb0d_0 conda-forge
wrapt 1.12.1 py38h1130bcf_2 conda-forge
xz 5.2.5 haf1e3a3_1 conda-forge
yaml 0.2.5 haf1e3a3_0 conda-forge
zlib 1.2.11 h7795811_1010 conda-forge
zstd 1.4.8 hf387650_1 conda-forge
```

Details about conda and system ( conda info ):

```
$ conda info
active environment : smithy-env
active env location : /Users/freddy.boulton/miniconda3/envs/smithy-env
shell level : 2
user config file : /Users/freddy.boulton/.condarc
populated config files :
conda version : 4.8.3
conda-build version : not installed
python version : 3.7.7.final.0
virtual packages : __osx=10.15.5
base environment : /Users/freddy.boulton/miniconda3 (writable)
channel URLs : https://repo.anaconda.com/pkgs/main/osx-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/r/osx-64
https://repo.anaconda.com/pkgs/r/noarch
package cache : /Users/freddy.boulton/miniconda3/pkgs
/Users/freddy.boulton/.conda/pkgs
envs directories : /Users/freddy.boulton/miniconda3/envs
/Users/freddy.boulton/.conda/envs
platform : osx-64
user-agent : conda/4.8.3 requests/2.23.0 CPython/3.7.7 Darwin/19.5.0 OSX/10.15.5
UID:GID : 1023994623:1834957490
netrc file : None
offline mode : False
```