Force garbage collection

        :param float threshold (optional): The threshold is optional, and
        will not change the default threshold.
        :rtype: boolean

def vacuum(self, threshold=0.3):
        '''
        Force garbage collection

        :param float threshold (optional): The threshold is optional, and
        will not change the default threshold.
        :rtype: boolean

        '''
        url = ("http://{master_addr}:{master_port}/"
               "vol/vacuum?garbageThreshold={threshold}").format(
            master_addr=self.master_addr,
            master_port=self.master_port,
            threshold=threshold)
        res = self.conn.get_data(url)
        if res is not None:
            return True
        return False