Do not load default service plugins if core plugin is not DB based

Some service plugins make the assumption that Neutron is running
with a datastore (e.g. revision and timestamps). As the datastore
setup is a responsibility of the Neutron core plugin, checking
that this is indeed true avoids errors for those plugins that do
not implement any DB backend (e.g. monolithic OpenContrail plugin).

Change-Id: I872fa6e3c3925e521150d79bba864101d9a5f648
Closes-bug: #1700651

neutron/db/db_base_plugin_v2.py

@@ -140,6 +140,9 @@ class NeutronDbPluginV2(db_base_plugin_common.DbBasePluginCommon,
     __native_pagination_support = True
     __native_sorting_support = True
 
+    def has_native_datastore(self):
+        return True
+
     def __new__(cls, *args, **kwargs):
         model_query.register_hook(
             models_v2.Port,

neutron/manager.py

@@ -179,7 +179,11 @@ class NeutronManager(object):
 
     def _get_default_service_plugins(self):
         """Get default service plugins to be loaded."""
-        return constants.DEFAULT_SERVICE_PLUGINS.keys()
+        core_plugin = directory.get_plugin()
+        if core_plugin.has_native_datastore():
+            return constants.DEFAULT_SERVICE_PLUGINS.keys()
+        else:
+            return []
 
     def _load_service_plugins(self):
         """Loads service plugins.

neutron/neutron_plugin_base_v2.py

@@ -410,3 +410,10 @@ class NeutronPluginBaseV2(base_services.WorkerBase):
         """
         return (self.__class__.start_rpc_state_reports_listener !=
                 NeutronPluginBaseV2.start_rpc_state_reports_listener)
+
+    def has_native_datastore(self):
+        """Return True if the plugin uses Neutron's native datastore.
+
+        .. note:: plugins like ML2 should override this method and return True.
+        """
+        return False