Two api tests for 'firewall insertion mode' feature

Two api tests for 'firewall insertion mode' feature

Some time ago the feature called 'fwaas insertion mode' was merged.
It allows to associate a firewall with routers.

The patchset adds two api tests:
* Create firewall assiciated with a router, add another router
to the firewall, remove old one
* Create firewall assoicited with a router, try to create new
firewall on the same router

Change-Id: I7c4d41189056ff6da47bc1173d3479183e58a173