[Ceilometer] Unit tests fail with invalid using of notification_topic option

[Ilya Tyaptin]

In Ib51e2839f9035d0cc0e3f459939d9f9003a8c810, oslo.messaging
is introducing a new group for the notifiction options. This
review is the minimum needed to support both old and
newer oslo.messaging versions. In other words, next
oslo.messaging release will break ceilometer unless we
work around for the new option group.

Trace:

Traceback (most recent call last):
  File "ceilometer/tests/unit/test_middleware.py", line 99, in test_targets
    targets = middleware.HTTPRequest(mock.Mock()).get_targets(self.CONF)
  File "ceilometer/middleware.py", line 55, in get_targets
    for topic in conf.notification_topics
  File "/home/jenkins/workspace/gate-ceilometer-python27/.tox/py27/local/lib/python2.7/site-packages/oslo_config/cfg.py", line 2009, in __getattr__
    raise NoSuchOptError(name)
oslo_config.cfg.NoSuchOptError: no such option in group DEFAULT: notification_topics

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/14689
Submitter: Pkgs Jenkins <email address hidden>
Branch: openstack-ci/fuel-8.0/liberty

Commit: 80ac6b8082effca291a128a5d29692700c1e7acf
Author: Davanum Srinivas <email address hidden>
Date: Tue Dec 15 11:55:28 2015

oslo.messaging option group/name change for notification topics

In Ib51e2839f9035d0cc0e3f459939d9f9003a8c810, oslo.messaging
is introducing a new group for the notifiction options. This
review is the minimum needed to support both old and
newer oslo.messaging versions. (In other words, next
oslo.messaging release will break ceilometer unless we
work around for the new option group)

Closes-bug: #1526288

Change-Id: I4765b3b9627983a245aa5521a85ad89e83ab8551
(cherry picked from commit 155d4fa894ea07264c03db73ba0828d3b96de1ae)