Don't provide IPv6 route when not defined in config

Summary:
On FreeBSD, /etc/rc.conf.d/routing/ipv6 was currently always provisioned,
with ipv6_defaultrouter="None" as content when pillar doesn't contain
any network ipv6_gateway variable.

Test Plan: Tested on router-001

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2333