Tenants API User Model - User Tests Parity.

1. Refactored a user model and test (tenants-api response models).
2. Fixed serialisation/deserialisation issues in the user class.
3. Fixed assertion issues in user_test.

Change-Id: I1f35ff6a029be4972f1ae259e594eacbb5828271
Tenants API Client - Role Tests Parity:

1. Added new paramater to tenants-api client.
2. Refactored assign/create tenant user role methods.
3. Added new section and property (tenants-api-admin) in Identity config.py.
4. Refactored creation of a tenant user.
5. Refactored removal of a tenant user role.
6. Deleted a section and property (tenants-api-admin) fron Identity config.py.
7. Added an admin extensions class to the constants model, to use it accross API clients.
8. Refactored tenants-api client and test.
9. Removed conftest file.
10. Rebased changes.

Change-Id: I3e52cdca465222095387a73dd95686cd462e950c
Merge "Tenants API User Model - User Tests Parity."
Merge "Tenants API Client - Role Tests Parity:"