Re-add baseurl lines, but using example domain
    
    This sorta reverts f30862ce, because it turns out various people
    (including me!) actually rely on these commented-out baseurl
    lines in various ways. I (in openQA tests) and at least one other
    person (hiredman on IRC), for e.g., use them as a base to modify
    by changing the host, but keeping the path component, to use a
    specific host that is known to always be a fast local one in a
    particular environment and avoid the overhead of the metalink/
    mirrorlist system.
    
    To avoid encouraging the use of the download.fedoraproject.org
    redirect system (as was the original intent), we replace the
    hostname with an example one, so the line clearly cannot be used
    as-is just by removing the #. We *could* also add a couple of
    comment lines specifically discouraging people from using baseurl
    unless they're very sure it's what they need, but I don't know
    if it's necessary.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>