Change warning messages of missing prefix for RH products
    
    Be clear that this will be removed in the future major release of the
    product (RHEL, CentOS, RHEV...).
    
    Print general message only once if any deprecation was found.
    
    Resolves: rhbz#1897657
    
    Suggested-by: Vladimir Slavik <vslavik@redhat.com>