Ability to packet-out packets via the openolt northbound NNI interface

The openolt driven broadcom based OLT (edgecore) needs the ability to receive packet-out events from an openflow controller and send those packets via the northbound NNI interface on the OLT device.  Currently this ability exists for sending packets southbound via an ONU port but needs to be extended for the northbound ports.   Presumably then for all ports.

The immediate purpose for this ability is so the DHCP relay openflow application can intercept DHCP packets from an ONU/RG, add the needed Option 82 information, and resend the modified DHCP packet back out the northbound NNI interface from the OLT device.

Below is an example of the dhcpl2relay config that would use this capability, where device of:000000000a4001cd is the openolt device and port 129 is the northbound 40/100Gb interface:

    "org.opencord.dhcpl2relay" : {
      "dhcpl2relay" : {
        "dhcpServerConnectPoints" : [ "of:000000000a4001cd/129" ]
      }
    }

Attached is a logfile that contains the relevant logging from onos, voltha, openolt driver, and bal_core_dist