Don't use Mock.called_once_with that does not exist

Class mock.Mock does not exist method "called_once_with()", it just
exists method "assert_called_once_with()". "called_once_with()" does
nothing because it's a mock object.

In OSC, only one place is still using "called_once_with()". Fix it.

Change-Id: Ib890e95d775c3fc43df80fa05c82d726e78cdac8
Partial Bug: 1544522

Author: Tang Chen

openstackclient/tests/common/test_commandmanager.py

@@ -100,6 +100,6 @@ def test_get_command_names(self):
             mock_pkg_resources,
         ) as iter_entry_points:
             mgr = commandmanager.CommandManager('test')
-            assert iter_entry_points.called_once_with('test')
+            iter_entry_points.assert_called_once_with('test')
             cmds = mgr.get_command_names('test')
             self.assertEqual(['one', 'cmd two'], cmds)