I've developed a library to use Arduino boards as an MqTT client with OpenHAB. You can download this library at GitHub: Frog (https://github.com/AchD/Frog).
I use this library since late 2017 and looking for someone who wants to test it, too. The focus is on building a reliable and easy-to-use library.
Please, send me your feedback.
with best regard,