Comment 7 for bug 1837199

Reviewed: https://review.opendev.org/702637
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=48f9b1c2ce798dc4a7f6b7b54dfaf38760d3a730
Submitter: Zuul
Branch: stable/train

commit 48f9b1c2ce798dc4a7f6b7b54dfaf38760d3a730
Author: Matt Riedemann <email address hidden>
Date: Mon Nov 18 11:36:20 2019 -0500

    Print help if nova-manage subcommand is not specified

    If a nova-manage command is executed without the -h option
    or a subcommand the user gets an ugly traceback. This is
    easily recreated:

      $ tox -e venv -- nova-manage db

    Make the action argument required, so we get a helpful error message
    instead.

      $ nova-manage db
      usage: nova-manage db [-h]
                          {archive_deleted_rows,ironic_flavor_migration,
                           null_instance_uuid_scan,online_data_migrations,
                           purge,sync,version}
                          ...
      nova-manage db: error: the following arguments are required: action

    Note that unit tests appear to be impossible for this, since doing so
    attempts to initialize an oslo.config 'CONF' singleton and this is
    something we've already done in 'nova.test' and can't do again.

    Change-Id: I24d03eed3aa3b882c49916938f4c25d76fd4e831
    Closes-Bug: #1837199
    Co-Authored-By: Stephen Finucane <email address hidden>
    (cherry picked from commit 51b0d4ca58f70fdd887dd053bfae1b2c918c33ba)