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-)
@thaJeztah See my comment here: https://github.com/dotcloud/docker/pull/7187/files#r15294914 -- I think it's fine to be concise in the general help, and then more detailed in `docker help register`. YMMV