Don't set a network manager if charms turned off managing neutron

hooks/nova_compute_utils.py

@@ -423,6 +423,8 @@ def network_manager():
     Obtain the network manager advertised by nova-c-c, renaming to Quantum
     if required
     '''
+    manager = None
+    if neutron_plugin_legacy_mode():
         manager = _network_config().get('network_manager')
         if manager:
             manager = manager.lower()