Add script to orchestrate multiple composes
    
    It may make sense to break a big compose into smaller chunks that can be
    done independently. This script allows describing the smaller parts,
    runs them with correct dependencies and arranges the result to look like
    a single big compose.
    
    All parts use the same koji event, that is either obtained from Koji, or
    from command line argument.
    
    JIRA: COMPOSE-2654
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>