Fix MR74, workaround for s4u-network-ns3-wifi

Fix MR74, workaround for s4u-network-ns3-wifi

Fix error in NS3 introduced at merge request #74.

Rebuild the routing table at each netzone seal.
In theory, this could be done once at the seal of the platform.
However, our current platform_created signal is called when the XML
parsing has finished. The seal occurs only later at the Engine::run.