Fix identity tests when domain specific drivers are enabled

This updates users and groups identity tests to check the existence
of domain specific drivers and, therefore, the existence of users
and groups on different domain drivers.

This adds a new feature flag to be used when domain specific drivers
are enabled.

Change-Id: Iedb470c51fa2174ab7651e6b7e22eff1f25f7aac
diff --git a/tempest/config.py b/tempest/config.py
index af9eefc..e78a07f 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -234,6 +234,12 @@
                 deprecated_reason="This feature flag was introduced to "
                                   "support testing of old OpenStack versions, "
                                   "which are not supported anymore"),
+    cfg.BoolOpt('domain_specific_drivers',
+                default=False,
+                help='Are domain specific drivers enabled? '
+                     'This configuration value should be same as '
+                     '[identity]->domain_specific_drivers_enabled '
+                     'in keystone.conf.'),
     cfg.BoolOpt('security_compliance',
                 default=False,
                 help='Does the environment have the security compliance '