docs: amend imports to remove autodoc mocking

[tiborsimko]

ReadTheDocs does not build the REST API documentation chapter properly:

• http://reana-job-controller.readthedocs.io/en/latest/restapi.html

probably beacuse of:

reading sources... [ 87%] license
reading sources... [100%] restapi

/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/checkouts/latest/docs/restapi.rst:7: WARNING: autodoc: failed to import function u'get_jobs' from module u'reana_job_controller.app'; the following exception was raised:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 551, in import_object
    __import__(self.modname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/app.py", line 32, in <module>
    import k8s
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/k8s.py", line 30, in <module>
    api = pykube.HTTPClient(pykube.KubeConfig.from_service_account())
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/pykube/config.py", line 22, in from_service_account
    with open(os.path.join(path, "token")) as fp:
IOError: [Errno 2] No such file or directory: '/var/run/secrets/kubernetes.io/serviceaccount/token'
/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/checkouts/latest/docs/restapi.rst:12: WARNING: autodoc: failed to import function u'create_job' from module u'reana_job_controller.app'; the following exception was raised:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 551, in import_object
    __import__(self.modname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/app.py", line 32, in <module>
    import k8s
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/k8s.py", line 30, in <module>
    api = pykube.HTTPClient(pykube.KubeConfig.from_service_account())
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/pykube/config.py", line 22, in from_service_account
    with open(os.path.join(path, "token")) as fp:
IOError: [Errno 2] No such file or directory: '/var/run/secrets/kubernetes.io/serviceaccount/token'
/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/checkouts/latest/docs/restapi.rst:17: WARNING: autodoc: failed to import function u'get_job' from module u'reana_job_controller.app'; the following exception was raised:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 551, in import_object
    __import__(self.modname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/app.py", line 32, in <module>
    import k8s
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/reana_job_controller-0.0.1.dev20170123-py2.7.egg/reana_job_controller/k8s.py", line 30, in <module>
    api = pykube.HTTPClient(pykube.KubeConfig.from_service_account())
  File "/home/docs/checkouts/readthedocs.org/user_builds/reana-job-controller/envs/latest/local/lib/python2.7/site-packages/pykube/config.py", line 22, in from_service_account
    with open(os.path.join(path, "token")) as fp:
IOError: [Errno 2] No such file or directory: '/var/run/secrets/kubernetes.io/serviceaccount/token'
looking for now-outdated files... none found

Locally, this works well because of autodoc_mock_imports directive:

$ git grep k8 docs/conf.py
docs/conf.py:autodoc_mock_imports = ['k8s', ]

It'll be good to amend imports in app.py to something like

from .k8s import foo, bar

so that autodoc_mock_imports directive wouldn't be necessary.