neutron2snabb updates related to L2TPv3 config features

Updates `neutron2snabb` for new requirements. See bottom of [Neutron API Extensions](https://github.com/SnabbCo/snabb-nfv/wiki/Neutron-API-Extensions).

Also updates `ipv6:pton'` to use `inet_pton(3)`.
Using `hexundump`. Throw error if input would be party ignored.

Note: `hexundump` pads from the right, as opposed to what we talked about. Doesn't matter though right?