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
diff --git a/tempest/tests/common/test_service_clients.py b/tempest/tests/common/test_service_clients.py
index 3de7aba..78e8e21 100644
--- a/tempest/tests/common/test_service_clients.py
+++ b/tempest/tests/common/test_service_clients.py
@@ -39,6 +39,7 @@
 from tempest.services.compute.json import security_group_default_rules_client \
     as nova_secgrop_default_client
 from tempest.services.compute.json import security_groups_client
+from tempest.services.compute.json import server_groups_client
 from tempest.services.compute.json import servers_client
 from tempest.services.compute.json import services_client
 from tempest.services.compute.json import tenant_usages_client
@@ -126,6 +127,7 @@
             quota_classes_client.QuotaClassesClient,
             nova_secgrop_default_client.SecurityGroupDefaultRulesClient,
             security_groups_client.SecurityGroupsClient,
+            server_groups_client.ServerGroupsClient,
             servers_client.ServersClient,
             services_client.ServicesClient,
             tenant_usages_client.TenantUsagesClient,