Support v6 in v4 tunnels

[Ryan Finnie]

https://askubuntu.com/a/992872 roughly works, but could be handled internally to netplan. My /etc/network/interfaces equivalent was this:

auto hetunnel
iface hetunnel inet6 v4tunnel
    address 2001:470:1f04:123::2
    netmask 64
    endpoint 72.52.104.74
    gateway 2001:470:1f04:123::1
    ttl 64
    up ip link set mtu 1480 dev $IFACE
    up service procps start

I'm thinking something like this:

network:
  version: 2
  renderer: networkd
  v4tunnels:
    hetunnel:
      addresses:
        - "2001:470:1f04:123::2/64"
      gateway6: "2001:470:1f04:123::1"
      mtu: 1480
      ttl: 64
      local-endpoint: 0.0.0.0
      remote-endpoint: 72.52.104.74

[Daniel Axtens (daxtens)]

Hi, I think this is also what LP: #1799487 asks for. While your bug got here first, that bug has got some more traction, so I've marked this as a duplicate of that. Please let me know if I'm wrong.