Move fake get_credentials methods to test_auth

fake_auth_provider needed fake_credentials which needs auth module which
needs keystone service clients for implementing fake get_credentials and
get_default_credentials. So this dependency blocked moving rest_client
module to tempest-lib because test_rest_client needs fake_auth_provider
like:

  rest_client
     -> test_rest_client
        -> fake_auth_provider
           -> fake_credentials
              -> auth
                 -> identity_client of v2 and v3
                    -> rest_client

So a dependency loop existed.
Both get_credentials and get_default_credentials of fake_auth_provider
were used in test_auth only.
So this patch moves fake get_credentials methods to test_auth for breaking
the dependency loop.

Change-Id: Id12341de52204e2c428e10b4b758b700b0fbab09
2 files changed