Fix install of docker on ARM nodes

[moul]

docker-machine create -d scaleway --scaleway-commercial-type=C1 fails at the docker install step, see below

Addresses #28

Full logs

---

I think we can use an alternative --engine-install-url when for C1 servers (see https://github.com/docker/machine/blob/8141874120abdc5c9056b6dbeb09bfb76a3eb290/libmachine/drivers/base.go#L88)
We should host this script on this repo and try to dynamically update the flag if --scaleway-commercial-type=C1.

---

But ideally, we should add a new provisioner (https://github.com/docker/machine/tree/master/libmachine/provision) that should also help @hypriot's driver (https://github.com/hypriot/docker-machine-hypriot)