code: def keystone_wait_for_propagation(self, sentry_relation_pairs,
                                        api_version):
          
          for (sentry, relation_name) in sentry_relation_pairs:
              rel = sentry.relation('identity-service',
                                    relation_name)
              self.log.debug('keystone relation data: {}'.format(rel))
              if rel.get('api_version') != str(api_version):
                  raise Exception("api_version not propagated through relation"
                                  " data yet ('{}' != '{}')."
                                  "".format(rel.get('api_version'), api_version))
docs: Iterate over list of sentry and relation tuples and verify that
             api_version has the expected value.
  
          :param sentry_relation_pairs: list of sentry, relation name tuples used
                                        for monitoring propagation of relation
                                        data
          :param api_version: api_version to expect in relation data
          :returns: None if successful.  Raise on error.