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

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