Updates for designs/neutron/*

Amended `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.

Also adds a smoketest for `neutron2snabb` to `designs/neutron/selfest.sh`.

More:

* Fixes a bug mentioned below ("only create configurations for local host").
* Implements atomic update-only-when-changed for `neutron-sync-agent`.
* Removes some obsolete configuration keys from `neutron2snabb` and `lib.nfv.config` (see `nd_light` for why `remote_mac` they became obsolete).