Bigbluebutton settings should be bigbluebuttonbn/xxx instead of bigbluebuttonbn_xxx

I noticed while working on CONTRIB-8399 that the bigbluebuttonbn settings were not really attached to the module as they should be.

Most of the settings are accessed using the config::get function which will look into eithe $CFG->bigbluebutton[$setting] or the $CFG->bigbluebuttonbn_XXX setting.

I find this confusing compared to the usual practice and would think that:

1. We would need to rename  bigbluebuttonbn_xxx into  bigbluebuttonbn/xxx in the settings.php class
2. Get somewhat rid of the config::get and config class in general. 
3. Get rid of $CFG->bigbluebuttonbn[$setting]  and replace it by another mechanism
4. Find a way to hide only a couple of settings instead of the whole set as the old mod/bigbluebutton/config.php. Not sure that we need to hide all settings and if it is still relevant.