Docker register command

* New API endpoint /register
* `docker login` doesn't prompt for password anymore, doesn't create accounts anymore
* new `docker register` command that uses the /register endpoint
* Clearer status messages (especially when using private registry servers)

Fixes #6902

Docker-DCO-1.1-Signed-off-by: Joffrey F <joffrey@docker.com> (github: shin-)
Not to my immediate knowledge, but I don't want to assume what private registries setups people might have in place already. "docker login" currently allows you to "register" to a private registry and some people may rely on it already (even though the official registry implementation doesn't do account management)