fix CI failure due to oslo.messaging 5.0.0
fix CI failure due to oslo.messaging 5.0.0

Recently, oslo.messaging does a refactor [1] which causes
get_batch_notification_listener returns a new type
BatchNotificationServer instead of previous MessageHandlingServer,
they have different attributes, so our test code which relies on
MessageHandlingServer fails in CI gate.

This patch makes test code not rely on oslo.messaging details.

[1] https://github.com/openstack/oslo.messaging/commit/990d894eaf0d18941b56d89bff2bca51de375640

Conflicts:
	ceilometer/tests/functional/test_notification.py

Change-Id: Id588bb4c22ac819617c87de021cdf580c4e0d0f5
Closes-Bug: #1580930
(cherry picked from commit 23d5c9f648d8352aa205fbe46ba9796a5424874f)

ityaptin/ceilometer,ityaptin/ceilometer,openstack/ceilometer,openstack/ceilometer