--swarm starts swarm on 2357 instead of 2375

[robhaswell]

Our new --swarm starts swarm on 2357 instead of 2375, which is the port published in the Swarm docs. This results in immediate user confusion.

[wallnerryan]

this is on purpose because swarm master talks to agent on 2375/2376 and swarm agent would not be able to run on the same node if they both used 2375/2376

In fact, swarm docs reference this as the manager port which is arbitrary and is up to the user.

from https://docs.docker.com/swarm/install-manual/
docker run -d -p <manager_port>:2375 swarm manage token://<cluster_id>