[V3] Renaming policy_client into policies_client

1. The policy_client.py file was renamed into policies_client.py
2. The PolicyClient class was renamed into PoliciesClient
3. The policy_client variables were renamed into policies_client

Partially implements blueprint consistent-service-method-names

Change-Id: I6335b0c7e41e3e7a410ac4c0912ac4416edd728d
5 files changed