new script: compose-create-next-dir
    
    A new script compose-create-next-dir is added, it can create empty
    directory for next compose, compose ID is determined by parsing the
    config file.
    
    get_compose_dir is copied from pungi with minor changes. Pungi is not
    intended to be used as a library, so not import pungi to use the
    function.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>