add destroy_containers_on_start (default false)

turning this on disables snapshotting, so that when Garden comes up it
will destroy any containers that it finds in the depot.
[develop]: Merge commit 'f113b236aa1f09f51176273a9665d471a7f1280e' into develop

* commit 'f113b236aa1f09f51176273a9665d471a7f1280e':
  add destroy_containers_on_start (default false)