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
Seems like @sttts didn't have many options on choosing a default:

> Not all regions allow private networks. Choose one which does, e.g. NY 2, Amsterdam 2 or Singapore 1 at the time of this writing (check the web UI for the current private network support).

I manually specified a region when I used the script. I can change the default to NY 2, though.