Fix ceilometer.alarm.notifier.trust import

Import ceilometer.alarm.notifier.rest instead of
ceilometer.alarm.notifier to be able to use the rest module.

Change-Id: I82eea5c2f3ec0a8c5f33307d41df45111788c50f
Closes-Bug: #1327818
Merge "Fix ceilometer.alarm.notifier.trust import"