notifications won't be requeued when event handling error

[Liusheng]

In ceilometer EventsNotificationEndpoint, if we didn't enable configured ack_on_event_error option, we expect the notification message will be requeued when handling error. see[1]. the messaging driver will handle two result oslo_messaging.NotificationResult.REQUEUE and oslo_messaging.NotificationResult.HANDLED to perform requeue or ack, see[2].
But in ceilometer the two result won't be returned, because the info() and error() method in EventsNotificationEndpoint will just return None.

[1] https://github.com/openstack/ceilometer/blob/master/ceilometer/event/endpoint.py#L78-L80

[2] https://github.com/openstack/oslo.messaging/blob/master/oslo_messaging/notify/dispatcher.py#L133-L138

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/217677

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/217677
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=c197f4793d3f2cb6ed37f6b8fe25a9e849665397
Submitter: Jenkins
Branch: master

commit c197f4793d3f2cb6ed37f6b8fe25a9e849665397
Author: liu-sheng <email address hidden>
Date: Thu Aug 27 19:54:42 2015 +0800

    Fix requeue process on event handling error

    The NotificationDispatcher in oslo_messaging is able to requeue
    notifications which cannot be successfully handled by endpoint. That need
    endpoint return a 'requeue' if exception happened on handling. In
    EventsNotificationEndpoint of ceilometer, the return value was always
    None.

    Change-Id: Iab10ca06766e1454d1d2338e72087b6bca7590b6
    Closes-Bug: #1489431