Disconnect from all MongoDB Clients.

def _disconnect(self):
        """Disconnect from all MongoDB Clients."""
        for cluster in self._clusters:
            if 'connection' in cluster:
                connection = cluster.pop('connection')
                connection.close()
        # Remove all attributes that are database names so that next time
        # when they are accessed, __getattr__ will be called and will create
        # new Clients
        for dbname in self._mapped_databases:
            self.__delattr__(dbname)
        self._mapped_databases = []