Simplify IPv6 routing for FreeBSD

Summary:
IPv6 default gateway route on FreeBSD needs to include interface.

NetBox -> pillar script ensuires device interface (e.g. igb0) is set
at interface level. That device is then always passed to the template:
interface: {{ interface['device'] }}

Ref T2295

Test Plan:

• route -6 add default fe80::a293:51ff:feb7:55ef%igb0
• Reboot WindRiver

Reviewers: Duranzed, yousra

Maniphest Tasks: T2295

Differential Revision: https://devcentral.nasqueron.org/D4040