Migrated test_credentials.py from tempest
This migrates the above files from tempest. This latest Change-Ids of
each file at the time of this migration of each fare the following:
* tests/test_credentials.py : I9e0dafd77ed0786b9a7455185d856fdc869daa76
to see the commit history for these files refer to the above sha1s in
the tempest repository
Depends-On: I9e0dafd77ed0786b9a7455185d856fdc869daa76
Change-Id: Ib24f3c26bcc97b9de635b50b6ffa10627cd881d4