Deprecate Docker Hub token based discovery

[nishanttotla]

It isn't recommended, nor a reliable way to do service discovery. We should start to deprecate it with the goal of completely removing it two releases from now.

cc @docker/core-swarm-maintainers

Related:
#2675
#2679

[nishanttotla]

We should add a line to the release saying that token based discovery will be deprecated after two releases.

We may not be able to print a warning during swarm create because it'll break the API.

[nishanttotla]

@dongluochen @aluzzardi any suggestions on how to add this deprecation? The most obvious place to do it would have been to print a warning message when creating a cluster using token discovery, but that might break use cases for people who rely on only a token being printed out on cluster create.

[aluzzardi]

Print the warning to stderr