MQTT Gateway retained flag

[Mihai]

I succeed to add the retained flag for the MQTT Gateway.
To access this flag, I have inserted few modification into the file MyGatewayTransportMQTTClient.cpp
The whole code and instructions for use are here: https://github.com/Mihai258/MySensors_MQTT_retained/tree/master

[hek]

Good!

Maybe you can clean it up and create a proper pull request?

A few PR notes:

• The define should probably follow the the rest of the sketch configs by prefixing it with MY_
• Adding it to /keywords.txt (gives highlighting it in the Arduino IDE)

[Mihai]

I did not work too much with github and still find it somehow a little bit annoying for me, but I was trying to do this.
I also have updated the new inserted constant as you advice.

The new definition should be moved to the MyConfig.h , but now it was easier to modify only one file.

My tests were done on MQTT Gateway made by Arduino Uno (clone) + Ethernet shield 5100 and it works very nice.