feat(image_to_vm): Add minimal GCE support.

Simply rolls our disk image into a tarball that can be passed to gcutil.
To actually be useful we need an OEM package to handle SSH keys.
fix(image_to_vm): Add GCE oem package
feat(release_util.sh): Support changing default upload path with env var
Merge pull request #139 from marineam/gce

GCE support