[neutron2snabb] Update the neutron2snabb design to handle the
current Neutron CSV dump format. Also includes support for
L2TPv3 configuration from the Neutron side.

This adds lib/json.jua containing a `decode' function for JSON
strings. Credit is due to Craig Mason-Jones' JSON4Lua.
[neutron2snabb] Only create configurations for local host.
Remove obsolete config keys from lib.nfv.config and
neutron2snabb .
[neutron-sync-agent] Only touch updated configurations in
SNABB_DIR and ensure consistency by using the atomic mv(1).
[neutron2snabb] Added designs/neutron/selftest.sh to smoketest
neutron2snabb.