Add support for deleting snapshots

You want to be using the HasSnapshot action here too, so that the user gets the correct feedback if there are no snapshots to delete - take a look at the rollback action for how this is implemented.

Checking that the named snapshot exists would be a good improvement for this action, both in this use case and for the rolback action, which currently will accept any old snapshot name and let the driver blow up if it doesn't exist.  This is a bit untidy.