Tenants API Service Model - Service Tests Parity.

 Refactored tenants-api service model and test as per service tests parity requirements.
 Changed "service" key to "OS-KSADM:service" in dict_to_obj and json_to_obj methods.
 Added an id_ attribute while constructing a service object in _dict_to_obj.

Change-Id: Id9d27fb29f8933b990e827fe5ba2222d24bc4911
Merge "Tenants API Service Model - Service Tests Parity."