Rename module robottelo.api.client

Rename module from robottelo.api.client to robottelo.api.requests. This rename
allows client code to use this wrapper library *exactly* like one would use the
normal requests library. That is, one can change this existing import:

    import requests

To this:

    from robottelo.api import requests

And everything else can be left untouched.

Also add module tests.robottelo.test_robottelo_api_requests to API docs.