Show canonical name in `docker ps`

After trying out the upcoming 1.3.0, I was pleased to see that https://github.com/docker/docker/pull/7686 made the default output of `docker ps` more compact. However, keeping the very first name could be confusing, as it might be the name resulting from a `--link` (`foo/bar`). @guywithnose @crosbymichael was that on purpose?

This patches makes sure that the canonical name is shown (which also happens to be the shortest), and avoid unnecessary processing of the names.