Support Docker Swarm Mode

Support as a new COE Docker Swarm Mode, based on Swarmkit, the new orchestration system included into the Docker Engine in 1.12+.

No additional configurations, like external Etcd, are required: They are already integrated into Swarm. So, the workflow should be significantly easier: Instanciate docker hosts, initialize the swarm on the first, join other managers, join workers.

The new ClusterTemplate should have the same properties of the current Swarm COEs, only type should change to avoid confusion, to "swarm-mode".

magnum cluster-template-create --name swarm-mode-cluster-template \
                           --image-id fedora-atomic-latest \
                           --keypair-id testkey \
                           --external-network-id public \
                           --dns-nameserver 8.8.8.8 \
                           --flavor-id m1.small \
                           --docker-volume-size 5 \
                           --coe swarm-mode

magnum cluster-create --name swarm-mode-cluster \
                  --cluster-template swarm-mode-cluster-template \
                  --master-count 3 \
                  --node-count 8

These commands create the template and instanciate a cluster with 3 managers and 5 workers.

Also, new images should be added and supported. I am thinking more to an approach like Docker Machine launched with the generic driver. In the beginning, we could start by supporting officially Ubuntu 16.04 rather than Fedora Atomic, and have some good hooks for adding the Docker hosts running on different distros provisioning easier, later.