Use ExtensionDescriptor from neutron-lib

ExtensionDescriptor has been rehomed to neutron-lib and is being
removed from neutron core.

See https://review.openstack.org/398113

Depends-On: I5a111e0033e518e39b3042f047ee9eebba77a0d5

Change-Id: Ida8c5302d5ad632d48ca79034ceb36b7637dd341

neutron_dynamic_routing/extensions/bgp.py

@@ -15,10 +15,10 @@
 #
 
 from neutron_lib.api import converters as n_conv
+from neutron_lib.api import extensions
 from neutron_lib.db import constants as db_const
 from neutron_lib import exceptions as n_exc
 
-from neutron.api import extensions
 from neutron.api.v2 import resource_helper as rh
 
 from neutron_dynamic_routing._i18n import _

neutron_dynamic_routing/extensions/bgp_dragentscheduler.py

@@ -17,6 +17,7 @@ import abc
 import six
 import webob
 
+from neutron_lib.api import extensions as api_extensions
 from neutron_lib import exceptions as n_exc
 from neutron_lib.plugins import directory
 from oslo_log import log as logging
@@ -97,7 +98,7 @@ class BgpDrAgentController(wsgi.Controller):
             request.context, kwargs['bgp_speaker_id'])
 
 
-class Bgp_dragentscheduler(extensions.ExtensionDescriptor):
+class Bgp_dragentscheduler(api_extensions.ExtensionDescriptor):
     """Extension class supporting Dynamic Routing scheduler.
     """
     @classmethod