Define ModularityLabel RPM header
    
    We intend to stop using the DistTag header and replace it with
    ModularityLabel.  Upstream RPM already supports it but it is not yet
    available in Fedora.
    
    Related Modularity ticket: https://pagure.io/modularity/issue/113
    
    Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1650286
    
    For the time being, we need to keep both.
    
    Signed-off-by: Petr Šabata <contyk@redhat.com>