Mrack cannot be installed via pip if gssapi is not present on a system

[pvoborni]

Tried build in readthedocs and it fails with:

$ /home/docs/checkouts/readthedocs.org/user_builds/mrack/envs/stable/bin/python -m pip install --exists-action=w --no-cache-dir -r requirements.txt
Collecting beaker-client>=27
  Downloading beaker_client-28.0-py3-none-any.whl (168 kB)
Collecting boto3
  Downloading boto3-1.15.16-py2.py3-none-any.whl (129 kB)
Collecting click
  Downloading click-7.1.2-py2.py3-none-any.whl (82 kB)
Collecting pyyaml
  Downloading PyYAML-5.3.1.tar.gz (269 kB)
Collecting asyncopenstackclient
  Downloading AsyncOpenStackClient-0.8.1.tar.gz (23 kB)
Collecting beaker-common
  Downloading beaker_common-28.0-py3-none-any.whl (42 kB)
Collecting PrettyTable
  Downloading prettytable-1.0.1-py2.py3-none-any.whl (22 kB)
Requirement already satisfied: Jinja2 in /home/docs/checkouts/readthedocs.org/user_builds/mrack/envs/stable/lib/python3.7/site-packages (from beaker-client>=27->-r requirements.txt (line 1)) (2.11.2)
Collecting gssapi
  Downloading gssapi-1.6.9.tar.gz (1.1 MB)
    ERROR: Command errored out with exit status 1:
     command: /home/docs/checkouts/readthedocs.org/user_builds/mrack/envs/stable/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-9rb3067q/gssapi/setup.py'"'"'; __file__='"'"'/tmp/pip-install-9rb3067q/gssapi/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-_ydqr8vm
         cwd: /tmp/pip-install-9rb3067q/gssapi/
    Complete output (13 lines):
    /bin/sh: 1: krb5-config: not found
    In distributed package, building from C files...
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-9rb3067q/gssapi/setup.py", line 87, in <module>
        link_args = shlex.split(get_output('krb5-config --libs gssapi'))
      File "/tmp/pip-install-9rb3067q/gssapi/setup.py", line 34, in get_output
        res = subprocess.check_output(*args, shell=True, **kwargs)
      File "/home/docs/.pyenv/versions/3.7.3/lib/python3.7/subprocess.py", line 395, in check_output
        **kwargs).stdout
      File "/home/docs/.pyenv/versions/3.7.3/lib/python3.7/subprocess.py", line 487, in run
        output=stdout, stderr=stderr)
    subprocess.CalledProcessError: Command 'krb5-config --libs gssapi' returned non-zero exit status 127.
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

In https://github.com/pythongssapi/requests-gssapi/issues/14 is said that gssapi is expected to be on a system when python-gssapi is being pulled in.

I think we should make beaker-client requirement optional (e.g. via extras_require ) / to be explicitly pulled in + beaker should not be registered as a provider if it is not present.