feat(contrib): add DigitalOcean provisioner booting CoreOS via kexec

This introduces a provisioning script for Deis on DigitalOcean droplets. As DigitalOcean
does not (currently) support CoreOS, we deploy CoreOS on top of Ubuntu 14.04 using kexec
on boot to switch to the CoreOS kernel and initrd. The state filesystem is a loop
mounted btrfs.

The CoreOS bootstrapping is heavily based on Levi Aul's code:

  https://gist.github.com/tsutsu/490f35f48897df0f5173

We don't use Debian 7.0 as base distribution because kexec is unstable on the Debian
kernel.

replaces #1159
NY2 is the default region selected for me when I create a droplet lately. I *really* hate to be a Big Dumb American, but I agree with @bacongobbler that SF1 (region 3) or NYC2 (region 4) is probably a better default for our users in general.