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].

Backport note, squashed with dependent patch:

  Don't override the original notification message
  (commit 4292ebd9d409a09b1e1f276ed447c82317cd14de)

Change-Id: I6dd56fbf31c29e26670068004f6fc935b91266f0
(cherry picked from commit 658b89941299370db67b34e8b26750f00c013ca9)
Closes-Bug: #1325172
Merge "Fix listener for update.start notifications" into stable/icehouse