Fix l3-agent network namespace IDs

L3-agent routers identify their namespaces as qrouter-<router-uuid>
not qrouter-<net-uuid>.  This makes perfect sense since routers
necesseritly span multiple networks

This patch fixes the desciption and changes the example namespace for
clarity.

Change-Id: I0f6578a7ca7d0d22236ce21f873bbc3bc366feae