Allow fe80:: range as a valid ipv6 gateway for public config

[coesensbert]

Description

Upon setting the public config of a node, allow fe80:: range as 'Gateway IPv6'. This can be a valid configuration if the local setup allows it. Example: fe80::1

Temporary workaround

ip netns exec public ip -6 route del default via 2a01:4f8:10a:710::1
ip netns exec public ip -6 route add default via fe80::1 dev public