Implement the proposal from
https://github.com/docker/docker/issues/24430#issuecomment-233100121
Removes acceptance policy and secret in favor of an automatically
generated join token that combines the secret, CA hash, and
manager/worker role into a single opaque string.
Adds a docker swarm join-token subcommand to inspect and rotate the
tokens.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
