Handle NotFoundException when deleting alarm

If delete a alarm which has been deleted from ceilometer
already, ceilometer client will throw a exception, and deletion
will fail, so we need to ignore NotFoundException when
deleting alarm.

Change-Id: I224c2de575106d3ce827d48853e0729b6bf9a221
(cherry picked from commit c661482b2edace9f8bded182548ba0db7a2cdafa)
Merge "Handle NotFoundException when deleting alarm" into stable/icehouse