Replace secrets with join tokens

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>
Upstream-commit: 2cc5bd33eef038bf5721582e2410ba459bb656e9
Component: engine

components/engine/docs/reference/commandline/node_inspect.md

@@ -41,7 +41,6 @@ Example output:
         "UpdatedAt": "2016-06-16T22:52:45.230878043Z",
         "Spec": {
             "Role": "manager",
-            "Membership": "accepted",
             "Availability": "active"
         },
         "Description": {