Allow 'copr' user to run 'sign' command
    
    See https://pagure.io/copr/copr/issue/636
    
    By default only root can run the `sign` command. This
    check is applied within obs-signd code. We need to
    allow regular user in the config, see `man sign.conf`.
    
    Also /usr/bin/sign is owned by root:obsrun with
    -rwsr-x--- hence we need to add a user to the obsrun group.