[neutron2snabb] Use hex-string as input to `l2tpv3_*_cookie'.
[keyed_ipv6_tunnel] Accept hex encoded string instead of plain string.
[neutron2snabb] Use `vif_details.zone_ip' instead of
`vif_details.l2tpv3_local_ip'.
[lib.protocol.ipv6] Call inet_pton(3) in `ipv6:pton'.
Merge pull request #283 from eugeneia/neutron2snabb-updates

neutron2snabb updates related to L2TPv3 config features