backend/gce: sleep between polling for status when shutting down
backend/gce: sleep between polling for status when shutting down

henrikhodne-abandoned/worker,travis-ci/worker,henrikhodne-abandoned/worker,Acidburn0zzz/worker,solarce/worker,Acidburn0zzz/worker,solarce/worker,travis-ci/worker,travis-ci/worker,henrikhodne-abandoned/worker,solarce/worker,Acidburn0zzz/worker