Solve issue of handling sigchld when dunning in a docker (#355)

If Patroni was started in a docker with pid=1 it will execute itself
with the same arguments. The original process will take care about init
process duties, i.e. handle sigchld and reap dead orphan processes.
Also it will forward SIGINT, SIGHUP, SIGTERM and some other signals to
the real Patroni process.

tests/test_utils.py

@@ -2,25 +2,7 @@ import unittest
 
 from mock import Mock, patch
 from patroni.exceptions import PatroniException
-from patroni.utils import reap_children, Retry, RetryFailedError, sigchld_handler, sleep
-
-
-def time_sleep(_):
-    sigchld_handler(None, None)
-
-
-class TestUtils(unittest.TestCase):
-
-    @patch('time.sleep', Mock())
-    def test_reap_children(self):
-        self.assertIsNone(reap_children())
-        with patch('os.waitpid', Mock(return_value=(0, 0))):
-            sigchld_handler(None, None)
-            self.assertIsNone(reap_children())
-
-    @patch('time.sleep', time_sleep)
-    def test_sleep(self):
-        self.assertIsNone(sleep(0.01))
+from patroni.utils import Retry, RetryFailedError
 
 
 @patch('time.sleep', Mock())