Set IPv6 gateway

Summary:
On FreeBSD, if we restart the routing service, the route table is flushed.

As the change 3f403b6b98a2 has used imperative commands to configure
the route, the routing service hasn't access to that information,
so we lost IPv6 connectivity.

Test Plan: service routing restart

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1097