backend: copr-repo: createrepo_c --outputdir must imply --baseurl
    
    We could specify "--basedir '../'" instead, but end-user dnf wouldn't
    know how to map the relative path to http:// locations (e.g.
    http://exmaple.com/results/user/project/chroot/devel/../some.rpm).
    
    So copy the same "hack" from createrepo() method, and specify --baseurl
    instead.  This required me to parse backend configuration again, so I
    reworked the logic so we do this on one place.
    
    Also we need to guess the ownername/dirname/chroot (same as we needed it
    in add_appdata()).  So do this more carefully, and on one place only.
    
    Fixes a bug in --devel detected by beaker tests.