Tenants API Client - User Tests Parity.

1. Refactored tenant responses model - obj_to_json method.
2. Refactored tenant client - create user for a tenant method.
3. Refactored tenant and client tests.

Change-Id: Ied4929f5387e56a33c7fbfd9e4a493c0e2442b78
Merge "Tenants API Client - User Tests Parity."