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)`.
(The value would have to be lengthened/shortened to the right size if not the right number of digits given, with a warning printed for shortening.)