RestClient: Don't hard code volume service name.

Updates the common/rest_client keystone_auth function so that
it doesn't hard code the 'service_name' for the volume service.

The motivation here is to allow Tempest to run in more environments
where the volume service name may not always be 'cinder'.

Change-Id: I3b892781929642fd42a3dd3b4ba8d44486bfe275
diff --git a/tempest/common/rest_client.py b/tempest/common/rest_client.py
index 8924fd3..cb18a9c 100644
--- a/tempest/common/rest_client.py
+++ b/tempest/common/rest_client.py
@@ -141,13 +141,7 @@
 
             mgmt_url = None
             for ep in auth_data['serviceCatalog']:
-                if ep["type"] == service and service != 'volume':
-                    mgmt_url = ep['endpoints'][self.region][self.endpoint_url]
-                    tenant_id = auth_data['token']['tenant']['id']
-                    break
-
-                elif (ep["type"] == service and ep['name'] == 'cinder' and
-                      service == 'volume'):
+                if ep["type"] == service:
                     mgmt_url = ep['endpoints'][self.region][self.endpoint_url]
                     tenant_id = auth_data['token']['tenant']['id']
                     break