Rename magnum events
Rename magnum events

Now Magnum has renamed its bay to cluster. So Ceilometer need to
change to receive the notification/event from Magnum. See
magnum/blob/master/magnum/conductor/utils.py#L104

Change-Id: I4be1790eaff960092af74aa811d612583a80d2ac

openstack/ceilometer,openstack/ceilometer