Enforces the use of Credentials (part2)

Multiversion auth part5

Refactor mangers, utils and test base classes to use Credentials instead
of username, password and tenant_name.
Makes changes to tests where needed - some of the tests create their own
managers.

Partially implements: bp multi-keystone-api-version-tests

Change-Id: If05f5704d90390362cebf45e2664f2bfbc72268d
Merge "Enforces the use of Credentials (part2)"