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 reconfigure traffic processes.
* `neutron2snabb`: Convert Neutron config into native Snabb Switch config file.
* `snabbnfv-traffic`: Load/reload network configuration and execute it with a suitable app network.

Based on the development by Rahul Mohan Rekha.