Only log IDs of callbacks in debug message

Logging the ID of the callback should be enough
since the ID contains the path to the callback.

Before:
Notify callbacks
[('neutron.services.segments.db._update_segment_host_mapping_for_agent-8784700722903',
  <function _update_segment_host_mapping_for_agent at 0x7fd58f74ed70>)]
for agent, after_create _notify_loop"

After:
Notify callbacks
['neutron.services.segments.db._update_segment_host_mapping_for_agent-8784700722903']
for agent, after_create _notify_loop"

TrivialFix

Change-Id: I01d886f2707479183d7ae9c530f5f3b7b3963340

neutron/callbacks/manager.py

@@ -139,7 +139,7 @@ class CallbacksManager(object):
         errors = []
         callbacks = list(self._callbacks[resource].get(event, {}).items())
         LOG.debug("Notify callbacks %s for %s, %s",
-                  callbacks, resource, event)
+                  [c[0] for c in callbacks], resource, event)
         # TODO(armax): consider using a GreenPile
         for callback_id, callback in callbacks:
             try: