docker-py is not returning the name of the registry
if already logged in. It can differ from the registry
specified by the user, which was return as registry.
- Ansible version of "docker login" CLI command
- Persists Docker registry authentification in .dockercfg (only login once - no need to specify credentials over and over again anymore)
- Works for all other docker-py based modules (docker, docker_images) as well as the Docker CLI client
