Currently the ceilometer-expirer doesn't delete expired AlarmChanges.
Remained AlarmChanges would be cause of wasted disk-space and slow response.
This patch aims to delete AlarmChanges.
This patch adds a new config option:
[database]
alarm_history_time_to_live = -1
Implements: blueprint delete-alarmhistory
DocImpact
Change-Id: Ib6bda6df720cf8514b621bfe13dd3acba941949f
9e1513c58a