Make port creation API idempotent regardless of HTTP method
Allow MAC to network ID association to be overwritten via API
Ensure port is greater than 0 when updating a port
Added check to start-up database sync to ensure ports are registered to existing networks
   - Work-around for q-dhcp bug that leaves remnant ports of deleted subnets
Merge pull request #12 from t-lin/savi-1.1

Changes for Asana task titled: updating ryu 1.1 with latest updates on mac duplicates