i think this should be enough info - lets try fixing the permission issue here that is probably why the plugin is not working - we can try two things here:
- try fixing the plugin dir permission - related issue: Failed to start - plugin folder permission denied - #2 by fadjar340
- or just use the default plugins dir to clone and build the mqtt-datasource plugin: /var/lib/grafana/plugins
can you look into these two things and report back? 
you can change the log level back to the default if needed to get the server running