Adds explicit decode for Python 3 (#2383)

Author: gguuss

iot/api-client/mqtt_example/cloudiot_mqtt_example.py

@@ -116,7 +116,7 @@ def on_publish(unused_client, unused_userdata, unused_mid):
 
 def on_message(unused_client, unused_userdata, message):
     """Callback when the device receives a message on a subscription."""
-    payload = str(message.payload)
+    payload = str(message.payload.decode('utf-8'))
     print('Received message \'{}\' on topic \'{}\' with Qos {}'.format(
             payload, message.topic, str(message.qos)))