Wait until all devices have failover status of 'In Sync'.

        :raises: UnexpectedClusterState

def _check_all_devices_in_sync(self):
        '''Wait until all devices have failover status of 'In Sync'.

        :raises: UnexpectedClusterState
        '''

        if len(self._get_devices_by_failover_status('In Sync')) != \
                len(self.devices):
            msg = "Expected all devices in group to have 'In Sync' status."
            raise UnexpectedDeviceGroupState(msg)