Use rpkg layouts for rpmdefines
    
    Earlier, `rpkg`'s layout mechanism was bypassed and rpmdefines were set in a
    fixed way by `fedpkg`. This breaks the new results dir feature from
    https://pagure.io/rpkg/pull-request/540
    
    Rpmdefines are changed to be read from selected layout here, including the new
    `rpmfilename` layout property.
    
    Signed-off-by: Otto Urpelainen <oturpe@iki.fi>