Strip the "_v2" suffix from API test filenames

Rename all API test modules by stripping off the "_v2" suffix from their
filenames. For example, this:

    tests/foreman/api/test_filter_v2.py

Is now this:

    tests/foreman/api/test_filter.py

Clean up the API test stubs. Give classes names in the form `FooTestCaseStub`,
strip off redundant decorators, add docstrings and ignore some irrelevant pylint
errors.

Update the API documentation. Make `docs/api/tests.foreman.api.rst` accurate.

Fix #1493.
Pylint has a name for every single error message it emits. You can see these with the `--list-msgs` option. I add these dashes to reflect the output of that option.

```
$ pylint --list-msgs | grep too-many-public-methods
:too-many-public-methods (R0904): *Too many public methods (%s/%s)*
```

I strip the space simply for the sake of being terse.