Share swarm advertisement path with engine

The format for node advertisement in swarm and docker engine (for
networking) is the same, and can be shared. Doing so allows us to start
toward removing the swarm-join container, and improves our HA situation
after engine discovery is run. Resolves docker#670.

Specifically, swarm was previously somewhat HA: if you set up 1
controller, 2 replicas and 1 node, then you could lose the controller
and still remain functional unless the swarm node needed to restart.
This change improves HA by removing the above failure mode as long as
engine discovery has been run and is up to date.

See also docker-archive/classicswarm#1368 which implemented this flag, and lays out the
future scenario we hope to achieve, and also docker#1032 which is our goal of
removing swarm join with automatical engine discovery.

kv_store.md

@@ -107,11 +107,11 @@ docker exec -it ucp-kv etcdctl \
  --ca-file /etc/docker/ssl/ca.pem \
  --cert-file /etc/docker/ssl/cert.pem \
  --key-file /etc/docker/ssl/key.pem \
- ls /docker/swarm/nodes
+ ls /docker/nodes
 
-/docker/swarm/nodes/192.168.122.196:12376
-/docker/swarm/nodes/192.168.122.64:12376
-/docker/swarm/nodes/192.168.122.223:12376
+/docker/nodes/192.168.122.196:12376
+/docker/nodes/192.168.122.64:12376
+/docker/nodes/192.168.122.223:12376
 ```