Reuse already existing groups from upstream tempest config

Tempest plugin tests uses the same config file/object from the
upstream tempest, therefore instead of registering the same group
again in plugin tests we should reuse already existing groups and
register only plugin specific option here.

Change-Id: I948fdcf20732b98d5ba5d34fe0352ea9cff59f91
Closes-Bug: #1659596
diff --git a/keystone_tempest_plugin/plugin.py b/keystone_tempest_plugin/plugin.py
index 9311409..76f7a9c 100644
--- a/keystone_tempest_plugin/plugin.py
+++ b/keystone_tempest_plugin/plugin.py
@@ -31,17 +31,13 @@
         return full_test_dir, base_path
 
     def register_opts(self, conf):
-        config.register_opt_group(conf, project_config.identity_group,
-                                  project_config.IdentityGroup)
-        config.register_opt_group(conf, project_config.identity_feature_group,
-                                  project_config.IdentityFeatureGroup)
+        config.register_opt_group(conf, config.identity_feature_group,
+                                  project_config.identity_feature_option)
         config.register_opt_group(conf, project_config.fed_scenario_group,
                                   project_config.FedScenarioGroup)
 
     def get_opt_lists(self):
-        return [(project_config.identity_group.name,
-                 project_config.IdentityGroup),
-                (project_config.identity_feature_group.name,
-                 project_config.IdentityFeatureGroup),
+        return [(config.identity_feature_group.name,
+                 project_config.identity_feature_option),
                 (project_config.fed_scenario_group.name,
                  project_config.FedScenarioGroup)]