[Yardstick-233]latency measurment by using pktgen-dpdk

[Yardstick-233]latency measurment by using pktgen-dpdk

VM A runs pktgen-dpdk as a traffic generator and is connected
back-to-back with VM B running testpmd for forwarding packets.

1. use linux expect to fetch the latency statics data.
2. fetch 10 samples of latency and calculate avg_latency.
3. use screen to keep test scripts running in the background.
4. add a function send_command() for screen in ssh.py

JIRA:YARDSTICK-233

Change-Id: I90ae64b3d198c79f159275ae35715eb284f05080
Signed-off-by: wu.zhihui <wu.zhihui1@zte.com.cn>