VOL-1202 OpenOLT - Incorporate Kpi extensions in openolt adapter    - Updated openolt_statistics to use voltha/extensions/kpi.. objects to comply with KpiEvent2 formatting and to publish to      Kafka.    -  Added minimal NNI and PON port classes to contain current kpi data from BAL driver to allow retrieval/ publising  of data using reactor.
VOL-1202 OpenOLT - Incorporate Kpi extensions in openolt adapter
   - Updated openolt_statistics to use voltha/extensions/kpi.. objects to comply with KpiEvent2 formatting and to publish to
     Kafka.
   -  Added minimal NNI and PON port classes to contain current kpi data from BAL driver to allow retrieval/ publising  of data using reactor.

   - Placed nni_port and pon_port into the openolt_statistics file.   Clean up unused imports.

   --- Change 3: Removed the extraneous intf_id reference in pon_pm_names

Change-Id: I54a2d25ac0d8709f97d652493ad6e7b3a8535f42

opencord/voltha,opencord/voltha,opencord/voltha,opencord/voltha,opencord/voltha