Make sure data directory is empty before trying to restore backup (#307)

We are doing number of attempts when trying to initialize replica using
different methods. Any of this attemp may create and put something into
data directory, what causes next attempts fail.
In addition to that improve logging when creating replica.

tests/test_postgresql.py

@@ -290,6 +290,7 @@ class TestPostgresql(unittest.TestCase):
         self.assertFalse(self.p.can_rewind)
 
     @patch('time.sleep', Mock())
+    @patch.object(Postgresql, 'remove_data_directory', Mock(return_value=True))
     def test_create_replica(self):
         self.p.delete_trigger_file = Mock(side_effect=OSError)
         with patch('subprocess.call', Mock(side_effect=[1, 0])):
@@ -307,6 +308,9 @@ class TestPostgresql(unittest.TestCase):
         with patch('subprocess.call', Mock(side_effect=Exception("foo"))):
             self.assertEquals(self.p.create_replica(self.leader), 1)
 
+        with patch('subprocess.call', Mock(return_value=1)):
+            self.assertEquals(self.p.create_replica(self.leader), 1)
+
     @patch.object(Postgresql, 'is_running', Mock(return_value=True))
     def test_sync_replication_slots(self):
         self.p.start()