simple_switch_13.py: Added ability to use buffer_id in FlowMod

This avoids to send two packets (FlowMod & PacketOut).

OF v1.3.1 (the specification most switch vendors implemented) mentions
in A.3.4.1 on page 65 that this is a valid way to avoid two packets.

Signed-off-by: Benny Eggerstedt <benjamin.eggerstedt@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>