Make cluster docker version configurable (with docker-ce)

[Spyros Trigazis]

We can use docker-ce in a system container like in [1].

I have tried this with docker-ce works pretty well so far.

We need to:
* Maintain a container image using docker-ce based on [1]
* add a software config or deployment to deploy docker-ce in a container
  only if the user has passes the appropriate label
* support container_infra_prefix [2] for swarm_fedora_atomic_v2

[1] https://github.com/projectatomic/atomic-system-containers/tree/master/docker-centos
[2] https://bugs.launchpad.net/magnum/+bug/1712810