Cleanup intermediate SVC_EXEC snapshots

Per feedback from the iteration demo, cleanup intermediate snapshots.  

Note that snapshots created by SVC_RUN commands are not cleaned up; to do so is a bigger task for which I am opening a ticket https://jira.zenoss.com/browse/CC-626
Plausibly yes.  To do so would involve more work to plumb it through the runner to the cli/cmd layer.  There was precedent for shelling out to serviced, so I went with the easier route.  (Admittedly the precedent is not for functions that are available in our api)