use current aiomqtt conventions

This appears to address kubealex#3 in as much as it works and the warning I
reported there is silenced. Note that this patch contains the PR I
submitted in kubealex#4.
	closes kubealex#3

Author: jpmens

plugins/event_source/mqtt.py

@@ -56,29 +56,28 @@ async def main(queue: asyncio.Queue, args: Dict[str, Any]) -> None:
             cert_reqs=validate_certs if validate_certs is not None else True,
         )
 
-    mqtt_consumer = aiomqtt.Client(
+    async with aiomqtt.Client(
         hostname=host,
         port=port,
         username=username,
         password=password,
         tls_params=tls_params if ca_certs else None,
-    )
-
-    await mqtt_consumer.connect()
-
-    try:
-        async with mqtt_consumer.messages() as messages:
-            await mqtt_consumer.subscribe(topic)
-            async for message in messages:
-                try:
-                    data = json.loads(message.payload.decode())
-                    await queue.put(data)
-                except json.decoder.JSONDecodeError:
-                    logger.exception("Decoding exception for incoming message")
-    finally:
-        logger.info("Disconneccting from broker")
-        mqtt_consumer.disconnect()
-
+    ) as mqtt_consumer:
+
+        try:
+            async with mqtt_consumer.messages() as messages:
+                await mqtt_consumer.subscribe(topic)
+                async for message in messages:
+                    try:
+                        try:
+                            data = json.loads(message.payload.decode())
+                        except json.decoder.JSONDecodeError:
+                            data = dict(payload=message.payload.decode())
+                        await queue.put(data)
+                    except json.decoder.JSONDecodeError:
+                        logger.exception("Decoding exception for incoming message")
+        finally:
+            logger.info("Disconneccting from broker")
 
 if __name__ == "__main__":
     """MockQueue if running directly."""