VOL-1072 OpenOLT - Minimal handling of alarm indication

OpenOlt driver is sending alarm indications to the adapter
which are currently not handled. Adding minimal handling
of the alarm to avoid logging of unknown indication.

Change-Id: Ieb7cb5e5097745a50aea8d415f2f4112feb72478
diff --git a/voltha/adapters/openolt/openolt_device.py b/voltha/adapters/openolt/openolt_device.py
index 7b3e272..317c2c8 100644
--- a/voltha/adapters/openolt/openolt_device.py
+++ b/voltha/adapters/openolt/openolt_device.py
@@ -191,6 +191,8 @@
                 elif ind.HasField('flow_stats'):
                     reactor.callFromThread(self.flow_statistics_indication,
                                            ind.flow_stats)
+                elif ind.HasField('alarm_ind'):
+                    self.log.info('alarm indication not handled')
                 else:
                     self.log.warn('unknown indication type')