Comment 0 for bug 1891888

In Aodh, alarm histories are not automatically removed and we need the following 2 configurations to have old alarm histories removed from database

 - alarm_history_time_to_live parameter is set
 - aodh-expirer command is executed periodically

Note that the similar expiration for Panko was previously introduces as a fix for another bug[1], and we should implement the same for aodh.

 [1] https://bugs.launchpad.net/tripleo/+bug/1746514