platform/gce: wait for image creation to finish on request
platform/gce: wait for image creation to finish on request

When scripting image uploads before a kola run, ongoing image creation
operations would cause kola to fail. This change makes image uploads
wait for the image create operation to complete before returning.

mjg59/mantle,glevand/coreos--mantle,dm0-/mantle,marineam/mantle,mischief/mantle,mjg59/mantle,glevand/coreos--mantle,mischief/mantle,dm0-/mantle,quantum/coreos-mantle,quantum/coreos-mantle,marineam/mantle