Since NAT is not a use case for ipv6, all connected ipv6 subnets
need to be advertised. There seems to be no way to add single rule
for all ipv6 subnets, therefore an allow rule will be added for all
ipv6 subnets.
Change-Id: Iaf89738f8d71a2f743ba699bebd5c8361c130a3c