fix(contrib/digitalocean): Update network device names for DigitalOcean

From https://coreos.com/releases/#379.0.0:

  Incompatible Change: QEMU/KVM based platforms change the virtio based network
  interface name from something like ens3 or ens4v1 back to the classic eth0, eth1,
  etc. names. This is because virtio device numbers (the v1 in ens4v1) are not any
  more predictable than the eth0 style names are.