Implement support of log.mode. (#3122)

There was one oversight of #2781 - to influence external tools that Patroni could execute, we set global `umask` value based on permissions of the $PGDATA directory. As a result, it also influenced permissions of log files created by Patroni.

To address the problem we implement two measures:
1. Make `log.mode` configurable.
2. If the value is not set - calculate permissions from the original value of the umask setting.

tests/test_file_perm.py

@@ -30,3 +30,6 @@ class TestFilePermissions(unittest.TestCase):
     def test_set_permissions_from_data_directory(self, mock_logger):
         pg_perm.set_permissions_from_data_directory('test')
         self.assertEqual(mock_logger.call_args[0][0], 'Can not check permissions on %s: %r')
+
+    def test_orig_umask(self):
+        self.assertIsNotNone(pg_perm.orig_umask)