code: def update_lun(self, add_luns=None, remove_luns=None):
          
          if not add_luns and not remove_luns:
              log.debug("Empty add_luns and remove_luns passed in, "
                        "skip update_lun.")
              return RESP_OK
          lun_add = self._prepare_luns_add(add_luns)
          lun_remove = self._prepare_luns_remove(remove_luns, True)
          return self.modify(lun_add=lun_add, lun_remove=lun_remove)
docs: Updates the LUNs in CG, adding the ones in `add_luns` and removing
          the ones in `remove_luns`