Added the Ubuntu image project for GCE.

The documentation:
https://cloud.google.com/compute/docs/operating-systems#ubuntu
shows that `ubuntu-os-cloud` is the proper project name.
Merge pull request #1676 from mbrukman/gce-ubuntu

Added the Ubuntu image project for GCE.
Merge branch 'master' of https://github.com/mitchellh/packer