Fix Sparck StreamingListener for monasca-analytics

From Spark 2.2, it is necessary to implement the onStreamingStarted
method. This patch adds the method.

Change-Id: If2d9ef8ffad879319bceacad92dcf8e3d125f63e

monasca_analytics/spark/streaming_context.py

@@ -65,6 +65,11 @@ class DriverStreamingListener(streaming.StreamingListener):
         logger.debug("Receiver has stopped: \n\t{}\n".format(
             receiverStopped))
 
+    @staticmethod
+    def onStreamingStarted(streamingStarted):
+        logger.debug("Streaming has been started: \n\t{}\n".format(
+            streamingStarted))
+
 
 def create_streaming_context(spark_context, config):
     """