Add OpenStack Neutron agents

Add scripts that connect Snabb Switch with OpenStack Neutron in directory `src/designs/neutron/`.

* `neutron-sync-master`: Snapshot the Neutron database into CSV files and make available via Git.
* `neutron-sync-agent`: Pull latest Neutron database from Git and use them to reconfigure traffic processes.
* `neutron2snabb`: Convert Neutron database CSV dump into config files for Snabb switch instances.
* `snabbnfv-traffic`: Create and execute an app network to realise a Neutron configuration.