docs: def update_volume(self, volumeID, metadata):
          '''update existing volume metadata
             the given metadata will substitute the old one
          '''
          log.debug('updating volume metadata: {}'.format(volumeID))
          rawVolume = self._req_raw_volume(volumeID)
          normalized = self.normalize_volume(rawVolume)
          normalized['metadata'] = metadata
          _, newRawVolume = self.denormalize_volume(normalized)
          self._db.modify_book(volumeID, newRawVolume)