Fix arch-override for container-build
    
    The buildContainer task of koji-containerbuild expects
    the task options "arch_override", not "arches". It also
    expects the value to be a space separated string. Both of
    these are also similar to how the regular build command
    is defined.
    
    Signed-off-by: Luiz Carvalho <lucarval@redhat.com>