usecase: add packet length option for traffic generator

Add a -l option to configure octet length for Ethernet packets produced
by VPP traffic generator.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Change-Id: Ifeccf14dbf646a5917f3ab5c358162018885e75a