[BUG] tls.create_self_signed_cert: fails with "C" longer/shorter than 2 characters

Description
Using tls.create_self_signed_cert with C being longer than 2 chars, it'll fail like this:

OpenSSL.crypto.Error: [('asn1 encoding routines', 'ASN1_mbstring_ncopy', 'string too long')]

Using tls.create_self_signed_cert with C being shorter than 2 chars, it'll fail like this:

OpenSSL.crypto.Error: [('asn1 encoding routines', 'ASN1_mbstring_ncopy', 'string too short')]

Setup
No specific prerequisites required

Steps to Reproduce the behavior
Execute:

salt-call --local tls.create_self_signed_cert \
    CN=SomeCommonName C=SomeCountry \
    ST=SomeState L=SomeLocation \
    O=SomeOrganization OU=SomeOrganizationalUnit \
    emailAddress=some@email.address.test cacert_path=/tmp/selfsigned tls_dir=tls

Alternatively, set C=C to trigger the string too short case.

This will result in the following unhandled exception/Traceback:

[ERROR   ] An un-handled exception was caught by salt's global exception handler:
Error: [('asn1 encoding routines', 'ASN1_mbstring_ncopy', 'string too long')]
Traceback (most recent call last):
  File "/usr/bin/salt-call", line 11, in <module>
    load_entry_point('salt==3002.2', 'console_scripts', 'salt-call')()
  File "/usr/lib/python3/dist-packages/salt/scripts.py", line 449, in salt_call
    client.run()
  File "/usr/lib/python3/dist-packages/salt/cli/call.py", line 58, in run
    caller.run()
  File "/usr/lib/python3/dist-packages/salt/cli/caller.py", line 112, in run
    ret = self.call()
  File "/usr/lib/python3/dist-packages/salt/cli/caller.py", line 219, in call
    ret["return"] = self.minion.executors[fname](
  File "/usr/lib/python3/dist-packages/salt/executors/direct_call.py", line 12, in execute
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/salt/modules/tls.py", line 1275, in create_self_signed_cert
    cert.get_subject().C = C
  File "/usr/lib/python3/dist-packages/OpenSSL/crypto.py", line 585, in __setattr__
    _raise_current_error()
  File "/usr/lib/python3/dist-packages/OpenSSL/_util.py", line 54, in exception_from_error_queue
    raise exception_type(errors)
OpenSSL.crypto.Error: [('asn1 encoding routines', 'ASN1_mbstring_ncopy', 'string too long')]
Traceback (most recent call last):
  File "/usr/bin/salt-call", line 11, in <module>
    load_entry_point('salt==3002.2', 'console_scripts', 'salt-call')()
  File "/usr/lib/python3/dist-packages/salt/scripts.py", line 449, in salt_call
    client.run()
  File "/usr/lib/python3/dist-packages/salt/cli/call.py", line 58, in run
    caller.run()
  File "/usr/lib/python3/dist-packages/salt/cli/caller.py", line 112, in run
    ret = self.call()
  File "/usr/lib/python3/dist-packages/salt/cli/caller.py", line 219, in call
    ret["return"] = self.minion.executors[fname](
  File "/usr/lib/python3/dist-packages/salt/executors/direct_call.py", line 12, in execute
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/salt/modules/tls.py", line 1275, in create_self_signed_cert
    cert.get_subject().C = C
  File "/usr/lib/python3/dist-packages/OpenSSL/crypto.py", line 585, in __setattr__
    _raise_current_error()
  File "/usr/lib/python3/dist-packages/OpenSSL/_util.py", line 54, in exception_from_error_queue
    raise exception_type(errors)
OpenSSL.crypto.Error: [('asn1 encoding routines', 'ASN1_mbstring_ncopy', 'string too long')]

Expected behavior
Either the self-signed key/certificate to be created or a reasonable error message pointing out invalid values of the passed parameters.

Versions Report

Salt Version:
          Salt: 3002.2

Dependency Versions:
          cffi: Not Installed
      cherrypy: 8.9.1
      dateutil: 2.7.3
     docker-py: Not Installed
         gitdb: 2.0.6
     gitpython: 3.0.7
        Jinja2: 2.10.1
       libgit2: 0.28.3
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 0.6.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: Not Installed
      pycrypto: Not Installed
  pycryptodome: 3.6.1
        pygit2: 1.0.3
        Python: 3.8.5 (default, Jul 28 2020, 12:59:40)
  python-gnupg: 0.4.5
        PyYAML: 5.3.1
         PyZMQ: 18.1.1
         smmap: 2.0.5
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.3.2

System Versions:
          dist: ubuntu 20.04 focal
        locale: utf-8
       machine: x86_64
       release: 5.4.0-56-generic
        system: Linux
       version: Ubuntu 20.04 focal

Start in salt/modules/tls.py at create_self_signed_cert, where the traceback shows the C subject field is assigned. Reproduce the issue with the provided salt-call command using one- and three-character C values, then ensure invalid country values produce a reasonable error or the certificate is created for valid input.