Separate server_groups_client from servers_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 server_groups_client from servers_client.

Partially implements blueprint consistent-service-method-names

Change-Id: Iffd5b32f8d04edee6302605dc68e03b130939ade
7 files changed