ceilometer.notification.NotificationService.stop() use attribute before
it is initialized, it will raise exception when start() fails.
This patch fixes it by putting initialization in __init__().
Note, also adds coordinator.stop() when service is stopped for
polling agents.
Change-Id: Ied2f086e1f50950b430095ae7ee89036fd4a89d9
Closes-Bug: #1418793