Add configuration 'retain' property for outgoing MQTT message

[diversit]

Fixed issue #2663

I did not find any documentation or example to update manually.
As far as I can see, the documentation gets updated automagically with the new connector attribute for outgoing messages.

Added a test to verify the send message contains the 'retain' property set to True, but cannot get this test to succeed.
It seems to go wrong somewhere in the Mqtt client code.
While debugging, I see the retain property is correctly set to true when the message is transformed into a byte[].
But when reconstructing the message from byte[] the retain setting seems to have been lost and therefore the value is False instead of the expected True.

Any idea how to fix this? I'm clueless.

[ozangunalp]

For the test, you need to start the consumer after the message has been produced. The best way to ensure that is to consume it once and check that retained flag is false, and then consume it again to check that retained flag is true.

[cescoffier]

The code change looks good, but yes, the test might be a bit brittle.

[ozangunalp]

@diversit I pushed a fix to the test you've added. Note that only the last message is retained with mqtt.