Comment 3 for bug 1389489

I propose a fix but I am still working on making UT pass, indeed, it change a lot of things in the existing tests.

I also use the is_router_enabled function already present and used here and there to dynamically check if the extension is really enabled in addition to the configuration flag, so it minimize a lot the code.

I basically take example on the trunk pages who already do this job: https://github.com/openstack/horizon/blob/master/openstack_dashboard/dashboards/project/trunks/panel.py#L31