use neutron-lib callbacks

The callback modules have been available in neutron-lib since commit [1]
and are ready for consumption.

As the callback registry is implemented with a singleton manager
instance, sync complications can arise ensuring all consumers switch to
lib's implementation at the same time. Therefore this consumption has
been broken down:
1) Shim neutron's callbacks using lib's callback system and remove
existing neutron internals related to callbacks (devref, UTs, etc.).
2) Switch all neutron's callback imports over to neutron-lib's.
3) Have all sub-projects using callbacks move their imports over to use
neutron-lib's callbacks implementation.
4) Remove the callback shims in neutron-lib once sub-projects are moved
over to lib's callbacks.
5) Follow-on patches moving our existing uses of callbacks to the new
event payload model provided by neutron-lib.callback.events

This patch implements #2 from above, moving all neutron's callback
imports to use neutron-lib's callbacks.

There are also a few places in the UT code that still patch callbacks,
we can address those in step #4 which may need [2].

NeutronLibImpact

[1] fea8bb64ba7ff52632c2bd3e3298eaedf623ee4f
[2] I9966c90e3f90552b41ed84a68b19f3e540426432

Change-Id: I8dae56f0f5c009bdf3e8ebfa1b360756216ab886

neutron/objects/rbac_db.py

@@ -15,14 +15,14 @@
 import abc
 import itertools
 
+from neutron_lib.callbacks import events
+from neutron_lib.callbacks import registry
 from neutron_lib import exceptions as lib_exc
 from six import add_metaclass
 from six import with_metaclass
 from sqlalchemy import and_
 
 from neutron._i18n import _
-from neutron.callbacks import events
-from neutron.callbacks import registry
 from neutron.common import exceptions as n_exc
 from neutron.db import _utils as db_utils
 from neutron.db import api as db_api