scripts: improve scripts for use cases

- Limit VPP's worker threads to use a single CPU core.
- Cleanup VPP instance if interface set up failed.
- Stop process of VPP and nginx by pid.
- Fix some typos.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>

Change-Id: Ib47c8150d93f865b38b71ad6b9357d607caab445