core_promote: upload gce image name to private build bucket
core_promote: upload gce image name to private build bucket

The build bucket should now be writable anyone performing releases,
allowing us to ensure that the original build and public buckets are
exactly in sync.

smilart/scripts,marineam/coreos-scripts,marineam/coreos-scripts,marineam/coreos-scripts,coreos/scripts,glevand/coreos--scripts,mjg59/scripts,mjg59/scripts,smilart/scripts,glevand/coreos--scripts,mjg59/scripts,coreos/scripts,smilart/scripts