Allow shutting down offline instance

If an instance is offline we definitely shouldn't start it up.
But shutting it down, should it be up by mistake is not "that" bad.
Still, we only allow it with --force, as it still performs an action on
an instance we shouldn't touch. This should make everybody happy.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>