usecase: add firewall usecase

Add usecase that uses VPP combined with Snort and Vectorscan
to create a demonstration of a firewall.

Signed-off-by: Paul Szczepanek <paul.szczepanek@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>

Change-Id: If1939d0a7f80d11d7c43803e59e2829097862dc1