[ADMIN_API][MGM]: Be nicer to Nova networks for deletes

Instead of deleting a bad instance instantly add to the DB with a DELETED
status so that the next run of the delete scheduler will wipe it out.

Change-Id: Ib5518ef51176729baa703eeb410911cd8437fa9c
Merge "[ADMIN_API][MGM]: Be nicer to Nova networks for deletes"