Add 'topdir' to all fedmsg/notifier messages.
    
    This will be useful for fedmsg recipients so they can construct URLs
    back to the compose.  They may need to know the compose topdir to
    distinguish between https://kojipkgs.fedoraproject.org/compose/rawhide/
    and https://kojipkgs.fedoraproject.org/compose/f24/
    
    Signed-off-by: Ralph Bean <rbean@redhat.com>