To generate a signed url, we need a private_key: https://developers.google.com/storage/docs/accesscontrol#Signed-URLs & https://github.com/GoogleCloudPlatform/gcloud-node/blob/v0.6.0/lib/storage/index.js#L267

We get this if a user provides a credentials object or path to a keyfile, but not in GCE or in the future, GAE. Is it going to be possible to get the private key in those environments?