Standby cluster can't have synchronous nodes (#3079)

`synchronous_standby_names` and synchronous replication only work on a real primary node and in case of cascading replication simply ignored by Postgres.
This fact was already addressed by `global_config.is_synchronous_mode`, but in case if in a standby cluster the `/sync` key in DCS is not empty, `patronictl list` and `GET /cluster` were falsely reporting some nodes as synchronous because this check was missing.

Close https://github.com/zalando/patroni/issues/3078

patroni/utils.py

@@ -952,7 +952,7 @@ def cluster_as_json(cluster: 'Cluster') -> Dict[str, Any]:
     for m in cluster.members:
         if m.name == leader_name:
             role = 'standby_leader' if config.is_standby_cluster else 'leader'
-        elif cluster.sync.matches(m.name):
+        elif config.is_synchronous_mode and cluster.sync.matches(m.name):
             role = 'sync_standby'
         else:
             role = 'replica'

tests/test_ctl.py

@@ -503,6 +503,14 @@ class TestCtl(unittest.TestCase):
         assert '2100' in result.output
         assert 'Scheduled restart' in result.output
 
+    def test_list_standby_cluster(self):
+        cluster = get_cluster_initialized_without_leader(leader=True, sync=('leader', 'other'))
+        cluster.config.data.update(synchronous_mode=True, standby_cluster={'port': 5433})
+        with patch('patroni.dcs.AbstractDCS.get_cluster', Mock(return_value=cluster)):
+            result = self.runner.invoke(ctl, ['list'])
+            self.assertEqual(result.exit_code, 0)
+            self.assertNotIn('Sync Standby', result.output)
+
     def test_topology(self):
         cluster = get_cluster_initialized_with_leader()
         cluster.members.append(Member(0, 'cascade', 28,