Comment 1 for bug 1479429

A smart solution would be to set "ended_at" on the resource, when the instance is deleted and filter out instances with this flags set, so the aggregation result will be always correct.

But we don't have such thing in the ceilometer dispatcher yet, so the proposed solution will be:
* Implements the "ended_at" thing for the futur
* Set 'percent_of_overlap=0' temporary in aodh for gnocchi alarms against "aggregated metrics across resources".