[V3] Separating groups_client from identity_client

As the qa-spec of consistent-service-method-names, we have decided
all service client modules will be separated into a single module
by each resource. So this patch separates groups_client from
identity_client.

Partially implements blueprint consistent-service-method-names

Change-Id: Iadb49298f41a6307d3b453b365d9b6be31b95781
7 files changed