'Retrieve from the config file the current authentication for a given URL, and
return the username, password'

@staticmethod
@conductor_only
def _get_registry_auth(registry_url, config_path):

    username = None
    password = None
    try:
        docker_config = json.load(open(config_path))
    except ValueError:
        return (username, password)
    if docker_config.get('auths'):
        docker_config = docker_config['auths']
    auth_key = docker_config.get(registry_url, {}).get('auth', None)
    if auth_key:
        (username, password) = base64.b64decode(auth_key).split(':', 1)
    return (username, password)
