Don't write to PGDATA if major version is not known (#2583)

It could happen that Patroni is started up before PGDATA was mounted. In this case Patroni can't determine major Postgres version from PG_VERSION file. Later, when PGDATA is mounted, Patroni was trying to create the recovery.conf even if the actual Postgres major version is newver than 12.

To mitigate the problem we double check that the `Postgresql._major_version` is set before writing recovery configuration or starting postgres up.

Close https://github.com/zalando/patroni/issues/2434

tests/test_bootstrap.py

@@ -112,6 +112,7 @@ class TestBootstrap(BaseTestPostgresql):
         config = {'users': {'replicator': {'password': 'rep-pass', 'options': ['replication']}}}
 
         with patch.object(Postgresql, 'is_running', Mock(return_value=False)),\
+                patch.object(Postgresql, 'get_major_version', Mock(return_value=140000)),\
                 patch('multiprocessing.Process', Mock(side_effect=Exception)),\
                 patch('multiprocessing.get_context', Mock(side_effect=Exception), create=True):
             self.assertRaises(Exception, self.b.bootstrap, config)