[py34] replace file() with open()

Running flake8 with python3 finds this problem:
./oslo_messaging/notify/_impl_routing.py:55:16: F821 undefined name 'file'
        return file(filename, 'r')

Change-Id: I5cd7475618f7a7532db770c2c5c61576a657fbb4

oslo_messaging/notify/_impl_routing.py

@@ -52,7 +52,7 @@ class RoutingDriver(notifier.Driver):
 
     def _get_notifier_config_file(self, filename):
         """Broken out for testing."""
-        return file(filename, 'r')
+        return open(filename, 'r')
 
     def _load_notifiers(self):
         """One-time load of notifier config file."""