Add debug possibility for nova-manage command

Sometimes if an error occurs it's good to have the ability to debug it.
Changes add flag --post-mortem to use post-mortem debugging.

It can be used as:
nova-manage --post-mortem <category> <command>

Change-Id: Ib8ab99fc6bc543500a862f414d7df763568b9b2d

nova/cmd/manage.py

@@ -1560,10 +1560,14 @@ category_opt = cfg.SubCommandOpt('category',
                                  help='Available categories',
                                  handler=add_command_parsers)
 
+post_mortem_opt = cfg.BoolOpt('post-mortem',
+                              default=False,
+                              help='Allow post-mortem debugging')
+
 
 def main():
     """Parse options and call the appropriate class/method."""
-    CONF.register_cli_opt(category_opt)
+    CONF.register_cli_opts([category_opt, post_mortem_opt])
     config.parse_args(sys.argv)
     logging.set_defaults(
         default_log_levels=logging.get_default_log_levels() +
@@ -1585,5 +1589,9 @@ def main():
         rpc.cleanup()
         return(ret)
     except Exception:
-        print(_("An error has occurred:\n%s") % traceback.format_exc())
+        if CONF.post_mortem:
+            import pdb
+            pdb.post_mortem()
+        else:
+            print(_("An error has occurred:\n%s") % traceback.format_exc())
         return(1)

nova/tests/unit/test_nova_manage.py

@@ -1427,9 +1427,20 @@ class TestNovaManageMain(test.NoDBTestCase):
     def test_error_traceback(self, mock_conf, mock_parse_args):
         with mock.patch.object(manage.cmd_common, 'get_action_fn',
                                side_effect=test.TestingException('oops')):
+            mock_conf.post_mortem = False
             self.assertEqual(1, manage.main())
             # assert the traceback is dumped to stdout
             output = self.output.getvalue()
             self.assertIn('An error has occurred', output)
             self.assertIn('Traceback', output)
             self.assertIn('oops', output)
+
+    @mock.patch('pdb.post_mortem')
+    @mock.patch.object(manage.config, 'parse_args')
+    @mock.patch.object(manage, 'CONF')
+    def test_error_post_mortem(self, mock_conf, mock_parse_args, mock_pm):
+        with mock.patch.object(manage.cmd_common, 'get_action_fn',
+                               side_effect=test.TestingException('oops')):
+            mock_conf.post_mortem = True
+            self.assertEqual(1, manage.main())
+            self.assertTrue(mock_pm.called)