Fix handling of values obtained from Keystone

The auth token and endpoint values obtained from Keystone used to be
silently ignored, because _get_client_with_token() function referred to
them using keyword argument names different from ones we called it with.

To fix it, moved `token` and `endpoint` to keyword arguments as
 `os_auth_token` and `tuskar_url` in _get_client_with_credentials().
Test modified.

Change-Id: I1011571de566ad18e31b905971f38105279561bd
Closes-Bug: #1246720