(gcr) simplify lookup of service account email (#1170)
(gcr) simplify lookup of service account email (#1170)

If the user now provides "gcr_enabled" as instance metadata, but does not
supply an email address, the default compute service account is used.

Also the user can optionally provide which GCR registry to use.
jtk54/spinnaker,duftler/spinnaker,duftler/spinnaker,spinnaker/spinnaker,duftler/spinnaker,Roshan2017/spinnaker,spinnaker/spinnaker,imosquera/spinnaker,jtk54/spinnaker,stitchfix/spinnaker,duftler/spinnaker,stitchfix/spinnaker,Roshan2017/spinnaker,ewiseblatt/spinnaker,imosquera/spinnaker,ewiseblatt/spinnaker,Roshan2017/spinnaker,spinnaker/spinnaker,skim1420/spinnaker,imosquera/spinnaker,skim1420/spinnaker,ewiseblatt/spinnaker,spinnaker/spinnaker,skim1420/spinnaker,jtk54/spinnaker,stitchfix/spinnaker,ewiseblatt/spinnaker,skim1420/spinnaker