Create some simple tests for path api/v2/hosts

Implement a few tests which check whether it's possible to GET or POST to the
path. Check for things like whether an HTTP 200 is generated when a GET request
is issued, that sane sets of data are returned, and that an HTTP 401 is
generated when an unauthorized HTTP request is made.

Add method `get_server_credentials` to module `robottelo.common.helpers`. Add
unit tests for this method.

Make a few pylint-related fixes.
Decorate method with `skip_if_bz_bug_open`
Add some API tests for "Model" Satellite entity