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

elements/heat-engine/install.d/20-heat-crontab

@@ -0,0 +1,6 @@
+#!/bin/bash
+# If https://bugs.launchpad.net/heat/+bug/1239377 is ever fixed we can
+# configure heat to purge automatically and remove this.
+cat <<EOF > /etc/cron.d/heat
+30 * * * * heat /opt/stack/venvs/heat/bin/heat-manage purge_deleted 1
+EOF