Refactor client models in python-barbicanclient

Switching to an OOP approach.
Also did some minor cleanup (removing old Verifications code and some
unused imports).

Change-Id: Ifb443eb0a3ca7bab570d9181b35f6fd5f09e727c
Implements: blueprint client-refactor-models
Merge "Refactor client models in python-barbicanclient"