Don't create permanent physical slot with name of the primary (#1392)

It is a regular issue that primary is recycling WALs when one of the replicas is down for a long time. So far there were only two solutions for such a problem and both of them are not perfect:
1. Increase `wal_keep_segments`, but it is hard to guess the good value.
2. Use continuous archiving and PITR, but it is not always possible.

This PR is introducing the way to solve the problem for static clusters, with a fixed number of nodes and names that never change. You just need to list the names of all nodes in the `slots` so the primary will not remove the slot when the node is down (not registered in DCS).
Of course, the primary will not create the permanent slot which is matching its own name.

Usage example: let's assume you have a cluster with nodes named *abc1*, *abc2*, and *abc3*.
You have to run `patronictl edit-config` and put the following snippet into the configuration:
```yaml
slots:
  abc1:
    type: physical
  abc2:
    type: physical
  abc3:
    type: physical
```

If the node *abc2* is the primary, it will always create slots for *abc1* and *abc3* even if they are not running, but will not create slot *abc2*.
Other nodes will behave the same.

Close #280

tests/test_postgresql.py

@@ -281,8 +281,8 @@ class TestPostgresql(BaseTestPostgresql):
     @patch.object(Postgresql, 'is_running', Mock(return_value=True))
     def test_sync_replication_slots(self):
         self.p.start()
-        config = ClusterConfig(1, {'slots': {'ls': {'database': 'a', 'plugin': 'b'},
-                                             'A': 0, 'test_3': 0, 'b': {'type': 'logical', 'plugin': '1'}}}, 1)
+        config = ClusterConfig(1, {'slots': {'test_3': {'database': 'a', 'plugin': 'b'},
+                                             'A': 0, 'ls': 0, 'b': {'type': 'logical', 'plugin': '1'}}}, 1)
         cluster = Cluster(True, config, self.leader, 0, [self.me, self.other, self.leadermem], None, None, None)
         with mock.patch('patroni.postgresql.Postgresql._query', Mock(side_effect=psycopg2.OperationalError)):
             self.p.slots_handler.sync_replication_slots(cluster)