Refactor Managers to a common base class

Multiversion auth part3

Refactor client managers to inherit from a common manager.Manager
class. Moves scenario base manager to clients.py.

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

Change-Id: Iddacbaa4593b7cb4d32538a5cade814751c180e0
Merge "Refactor Managers to a common base class"