Test Neutron host routes and gateway with IPv6

This commit adds IPv6 testing to:

cloudroast/networking/networks/scenario/test_l2_hostroutes_gateway.py

This script tests the host routes and gateway functionallity in Neutron.

This test is part of phase II of the 'Exposing Neutron' project.

Change-Id: Ic38cf829d86328939d171a9b3f25a021c229a468