Add purge_deleted cron job to heat-engine

Heat-engine is currently only single-host, so this works in that model.
When heat-engine adds scale-out, we will probably want to do something
to make sure that only one of these runs, though it won't hurt to just
run a few of them at one time.

Change-Id: I2fe92489404e7ae490d78323d944703ec7d19007
Merge "Add purge_deleted cron job to heat-engine"