Possible fix to message encoding issue (GoogleCloudPlatform#1990)

It throws an exception when trying to parse to JSON. Could use a second opinion on this solution. Original Codelab instructions available [here](https://codelabs.developers.google.com/codelabs/cloud-iot-core-overview/index.html?index=..%2F..index#0).

Author: CarlosMendonca

iot/api-client/end_to_end_example/cloudiot_pubsub_example_mqtt_device.py

@@ -122,7 +122,7 @@ def on_subscribe(self, unused_client, unused_userdata, unused_mid,
 
     def on_message(self, unused_client, unused_userdata, message):
         """Callback when the device receives a message on a subscription."""
-        payload = message.payload
+        payload = message.payload.decode('utf-8')
         print('Received message \'{}\' on topic \'{}\' with Qos {}'.format(
             payload, message.topic, str(message.qos)))
 

iot/api-client/end_to_end_example/cloudiot_pubsub_example_server.py

@@ -152,7 +152,7 @@ def callback(message):
             subscribed topic.
             """
             try:
-                data = json.loads(message.data)
+                data = json.loads(message.data.decode('utf-8'))
             except ValueError as e:
                 print('Loading Payload ({}) threw an Exception: {}.'.format(
                     message.data, e))