power down the OpenThreadWpan

def powerDown(self):
        """power down the OpenThreadWpan"""
        print '%s call powerDown' % self.port
        if self.__sendCommand(WPANCTL_CMD + 'setprop Daemon:AutoAssociateAfterReset false')[0] != 'Fail':
            time.sleep(0.5)
            if self.__sendCommand(WPANCTL_CMD + 'reset')[0] != 'Fail':
                self.isPowerDown = True
                return True
            else:
                return False
        else:
            return False