Got an error when executing `nova-manage db`

[Eric Xie]

Description
===========
When i used `nova-manage db` and enter, got an error.

Steps to reproduce
==================
1. Execute `nova-manage db`

Expected result
===============
Output helper info like as queens
# nova-manage db
usage: nova-manage db [-h]
                      {archive_deleted_rows,ironic_flavor_migration,null_instance_uuid_scan,online_data_migrations,sync,version}
                      ...
nova-manage db: error: too few arguments

Actual result
=============
An error has occurred:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/oslo_config/cfg.py", line 3179, in __getattr__
    return getattr(self._conf._namespace, name)
AttributeError: '_Namespace' object has no attribute 'action_fn'

Environment
===========
# git log
commit ab6834145f3fe1d33ce7f292727a6bc2be50efd9 (HEAD -> stable/train, origin/stable/train)
Merge: 66585e8af1 821506a50c
Author: Zuul <email address hidden>
Date: Mon Oct 21 18:50:20 2019 +0000

    Merge "Fix exception translation when creating volume" into stable/train