Now the RPi based on a new install of Raspbian Jessie, OpenHab 1.8.0 and Mosquitto is up and running and the ESP8266 MQTT client does connect without any problems. I have not changed anything in the code but Mosquitto is now based on the RPi repository for Jessie and not the mosquito_wheezy one. That seems to have fixed it so those versions are likely different but I have not put any time into checking this.
@4994james or just use internal vcc measurement as described on the same page. That saves you the voltage divider.
I wrote a convenient library to measure vcc internally : https://github.com/Yveaux/Arduino_Vcc