Adapter IP Address Assignment for Adapter to OLT Communication

For adapters that communicate with OLT hardware via IP the adapter requires an IP address.
VOLTHA core should provide a configurable IP address pool from which adapters are assigned an address via a request to voltha core via a core provided API.
The '--pon-subnet' parameter was recently added to the voltha command line. This is the obvious network from which adapter addresses could be assigned.