By creating the file org.eclipse.smarthome.mqttbroker.cfg in the service folder (/usr/local/etc/openhab2/services in my case, chown to openhab), with the contents
name=mosquitto
host=192.168.1.69
secure=false
port=1883
username=
password=
retain=true
I was able to just create the MySensors MQTT gateway (using paperui) and it stayed online (no mqtt broker defined, but its addon is installed). I had the impressions that with the latest versions it should be possible to do everything in the PaperUI, and that the broker would use the mqtt2 addon (but in paper ui, service, config, mqtt tab, mqtt system broker connection, manage->plus button -> expert mode, I get to see the contents of the cfg file above)
But unfortunately, it seems that the discovery services enters a endless loop (maybe I need to update my arduinos sketch to the latest mysensors code?) and I don't get any auto discovered things. If I trigger a new discovery it will stop the previous one and start again.
Finally, I was able to manually create a thing, and link items on it. Eventually my battery level and humidity were displayed in the paper ui control page (after some weird delay, not updating once it shows in the log. the temperature didn't show up though).
Not sure if my understanding is off (or paperui is limited in this sense) but in the "thing" creation page I only get to specify one child id. Then my multi sensors/value nodes would need to be define in multiple things? As I can't properly define channels for each child id, and I'm not sure what var1-5 are all about.
I'll try defining things/channels/items directly in files in the following days, because so far I couldn't enjoy most of the benefits of this new version (auto discovery, logical grouping of physical device in "things", questionable "do-it-all in paper ui"), and I'm almost considering using mqtt1 binding in the new install 
