Add method `get_server_credentials`

New method simply returns a tuple of ('username', 'password'). It's simple, but
more convenient than reading values from the config incrementally. Also, add
unit tests for the new method.
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.
Decorate method with `skip_if_bz_bug_open`
Create some simple tests for path api/v2/models

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.