usecase: add combined firewall usecase

Add a use case for next generation firewall (ngfw) with
a combination of L2 acls, L3 acls, and snort. Example
traffic generators hitting each case and a benign case included.

Signed-off-by: Robin Shapley <robin.shapley@arm.com>
Change-Id: Ic5f0834be141b5cabe6d8c4c63506ebb4a43b2ce