Tenants API User Model - Role Tests Parity

1. Refactored the tenants-api user responses model.
2. Fixed serialization/deserialization issues for roles tests parity.
3. Added and tested _obj_to_json method in user response model.

Change-Id: I7f2209678224aee5cc86bc4875417eae41e6c505
Merge "Tenants API User Model - Role Tests Parity"