[RFC] Add "docker service rollback" subcommand

[thaJeztah]

docker 1.13 added a --rollback flag to docker service update (moby/moby#26421) to allow manually rolling back a service to a previous version.

This flag cannot be combined with other flags on the service update subcommand, which is confusing (moby/moby#33444)

Looking at this functionality, I think this flag is essentially a subcommand "disguised" as a flag, and I suggest to add a rollback subcommand;

docker service rollback <service-name|service-id>

which would be the equivalent of

docker service update --rollback <service-name|service-id>

Given that this functionality is already present in the API, this can be implemented as a cli-only change.

ping @aaronlehmann @dnephin @tiborvass

[aaronlehmann]

It used to be possible to combine --rollback with other flags. Now that it isn't, I'm fine with adding the subcommand.

[dnephin]

SGTM