feat(udev): Add short disk by-id dev links for GCE

Standard GCE images setup `/dev/disk/by-id/google-<disk-name>` links for
attached storage. The default rules only provide the much-more verbose
`/dev/disk/by-id/scsi-0Google_PersistentDisk_<disk-name>`.

Fixes https://github.com/coreos/bugs/issues/13