[V3 identity] Renaming service_client into services_client

1. The service_client.py file was renamed into services_client.py
2. The ServiceClient class was renamed into ServicesClient
3. The service_client variables were renamed into services_client

Partially implements blueprint consistent-service-method-names

Change-Id: I766841b19ea33426371f53651d55bc31feb6790c
7 files changed