Add support to use existing data disks

[carmstrong]

I created https://deis.blob.core.windows.net/releases/deis-v1_1_1.vhd which is a data disk mounted as /var/lib/docker on a CoreOS host that has Deis v1.1.1 images pre-cached in the Docker graph.

If the azure-coreos-cluster script has a flag for, say, --data-disk-url, we could attach this disk on boot instead of creating a blank data disk. If we drop format-ephemeral.service from deis-cloud-init-with-disk.yml (but still mount it with var-lib-docker.mount), new machines coming up should already have Deis v1.1.1 cached locally and wouldn't need to pull it from DockerHub. This should significantly speed up Deis deployments on Azure.

[carmstrong]

I modified the AWS provision script in Deis to use our shared cloud-config file but dynamically add the units to format and mount the /var/lib/docker volume: deis/deis#2803

You should be able to steal most of that and do this pretty easily.