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>.
Merge pull request #106 from marineam/gce

feat(udev): Add short disk by-id dev links for GCE
feature(network-online.target): ensure IP routable before enabling

This adds a new service to delay network-online.target until the box has a routable IP.