Make Sphinx happy

Do not attempt to auto-generate API docs for the non-existent
`robottelo.api.apicrud` module. Fix a module reference.
Skip known-failing tests

Skip tests that are known to fail due to BZ 1154156.

    $ nosetests tests/foreman/api/test_multiple_paths.py -m _Host
    ..S.S.....S.....SSSS
    ----------------------------------------------------------------------
    Ran 20 tests in 48.694s

    OK (SKIP=7)

Make method `Host.create` subnet a nested hash of parameters, e.g. `{'host':
{...}}`.
Merge pull request #1586 from Ichimonji10/docs

Make Sphinx happy
Merge pull request #1588 from Ichimonji10/create-host

Skip known-failing tests
Trim down .gitignore file

Only ignore files and directories that are relevant to this project.
Unnecessarily ignoring files can make troubleshooting especially hard in
situations where a troublesome file is invisible to git.