Add --ephemeral flag for using data disk for Docker storage

This pull request corresponds to [this PR](https://github.com/kenperkins/coreos-cluster/pull/9) in kenperkins/coreos-cluster.  

A common issue running CoreOS in Rackspace's cloud vms is that /var/lib/docker runs out of space pretty quickly on the system disk because the Docker btrfs partition is not allocated a whole ton of space.