Refactor test code

Several unit tests override or modify a variety of objects, typically for
mocking purposes. Back up several of those objects before they are overridden or
modified, then restore those objects to their existing state after the relevant
test has finished. This prevents the test code in one location from affecting
the state of test code in other locations.

Remove function `robottelo.factory._call_client_post`. This wrapper is
unnecessary, as it wraps `robottelo.api.client.post`, which is also in the
codebase. Test code can just as easily override one as the the other, and having
less code lying around means less of a maintenance burdern.

Add several pylint directives.