Convert "default" to GCE-specific default image
Convert "default" to GCE-specific default image

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