docs: def _set_mongodb_host_val(key, default, mongodb_host, mongodb_defaults):
      """
      Set a value in a 'cascade' fashion for mongodb_host[key]
  
      Within 'mongodb', as a last resort, its hardcoded default value is going to
      be picked.
  
      :param key: key name
      :param default: default last resort value
      :param mongodb_host: mongodb 'host' entry
      :param mongodb_defaults: mongodb 'defaults' dict
      """
      # If mongodb_host[key] is not already set, its value is going to be picked
      # from mongodb_defaults[key]
      if key not in mongodb_host:
          if key in mongodb_defaults:
              mongodb_host[key] = mongodb_defaults[key]
          else:
              # BUT, if also mongodb_defaults[key] doesn't exist
              # the value picked is going to be 'default' as last resort
              mongodb_host[key] = default
          if key != 'user_name' and key != 'password':
              log.msg_debug("Setting default '{key}'='{value}' "
                            .format(key=key, value=mongodb_host[key]))