fix(contrib): give /var/lib/docker 30GB on DO

This bumps our partitions on DigitalOcean to 30GB, which is more
suitable for CoreOS. An external block storage would be more suitable,
but alas...

This also has a side effect of restricting the size of the VM to 2GB+
as per the spec on DigitalOcean droplets.
Merge pull request #1427 from deis/1409-docker-partition

fix(contrib): give /var/lib/docker 30GB on DO