feat(coreos-base/oem-gce): Add a message about the cloud sdk container

Since it isn't simple for us to provide the Python based Google Cloud
SDK tools users should be directed to the Docker container instead.

Also fix the bug report URL, not that anyone looks at it... :)
Merge pull request #643 from marineam/gce

feat(coreos-base/oem-gce): Add a message about the cloud sdk container
bump(coreos-cloudinit): upgrade to v0.7.3
Merge pull request #646 from jonboulle/master

bump(coreos-cloudinit): upgrade to v0.7.3
Merge branch 'master' of https://github.com/coreos/coreos-overlay