Log in to one instance.

def login(ctx, host, scenario_name):  # pragma: no cover
    """ Log in to one instance. """
    args = ctx.obj.get('args')
    subcommand = base._get_subcommand(__name__)
    command_args = {
        'subcommand': subcommand,
        'host': host,
    }

    s = scenarios.Scenarios(
        base.get_configs(args, command_args), scenario_name)
    for scenario in s.all:
        base.execute_subcommand(scenario.config, subcommand)