backend.InstanceShutdown: small cleanup

1) unhardcode the timeout, abstracting it in a constant
2) Use time.time() rather than hiding the timeout in a range()
3) call hyper.StopInstance multiple times
   -- currently all hypervisors just ignore all calls but once
4) Use hyper.ListInstances() rather than GetInstanceList([hv_name])
   -- it's cheaper :)
5) Change the final message to "forcing" from "using destroy"

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>