[live-media] Use subvariants in logging
    
    Print the subvariant together with variant UID and arches. This should
    make the global log easier to understand.
    
    The output of koji command should now be split into multiple files in
    logs/{arch}/livemedia-{variant}-{subvariant}.{arch}.log instead of
    having them all in one file.
    
    The tests were updated to use more sensible value for the subvariant
    field.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>