Update neutron2snabb to handle updated Neutron CSV dump format

Includes support for L2TPv3 configuration from the Neutron side.

This adds `lib.json:decode` for parsing JSON strings. Credit is due to Craig Mason-Jones' JSON4Lua.
We should do additional filtering somewhere here:
The database configuration includes records for every compute host (potentially hundreds) but we only want to generate config files for the local traffic processes on our node.

So we should have a check along the lines of:

```
if vif_details.zone_host == myhostname then
   ...
end
```

where myhostname is the unix host name of the machine.