shared.unattended: Do not use python script in RHELs' kickstart

We just run some bash command in python. So directly run these
command in post script.
Signed-off-by: NFeng Yang <fyang@redhat.com>
Acked-by: NXiaoqing Wei <xwei@redhat.com>
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>