Fix listener for update.start notifications

When listening for update.start, ceilometer-notification-agent
will raise a KeyError exception. This is because, the "id" in notification
message is in message['payload'] not the message['payload'][resource].

Change-Id: I6dd56fbf31c29e26670068004f6fc935b91266f0
Closes-Bug: #1325172
Merge "Fix listener for update.start notifications"