[V3] Renaming region_client into regions_client

1. The region_client.py file was renamed into regions_client.py
2. The RegionClient class was renamed into RegionsClient
3. The region_client variables were renamed into regions_client

Partially implements blueprint consistent-service-method-names

Change-Id: Ic80fe18377ce7a1f02068e78e2a90d9505ed3df3
5 files changed