cell_v2 update_cell command don't allow default value

[jichenjc]

devstack built 5 days before ,the param should be optional but it's required now

jichen@ubuntu1604:~$ nova-manage cell_v2 update_cell --cell_uuid b674a67e-a4a3-414a-9507-f08407686d37
Updates the properties of a cell by the given uuid.

        If the cell is not found by uuid, this command will return an exit
        code of 1. If the properties cannot be set, this will return 2.
        Otherwise, the exit code will be 0.

        NOTE: Updating the transport_url or database_connection fields on
        a running system will NOT result in all nodes immediately using the
        new values. Use caution when changing these values.

Traceback (most recent call last):
  File "/opt/stack/nova/nova/cmd/manage.py", line 1598, in main
    fn, fn_args, fn_kwargs = cmd_common.get_action_fn()
  File "/opt/stack/nova/nova/cmd/common.py", line 160, in get_action_fn
    _("Missing arguments: %s") % ", ".join(missing))
Invalid: Missing arguments: name, transport_url, db_connection

usage: nova-manage cell_v2 update_cell [-h] --cell_uuid <cell_uuid>
                                       [--name <name>]
                                       [--transport-url <transport>]
                                       [--database_connection <database>]

optional arguments:
  -h, --help show this help message and exit
  --cell_uuid <cell_uuid>
                        The uuid of the cell to update.
  --name <name> Set the cell name.
  --transport-url <transport>
                        Set the cell transport_url. NOTE that running nodes
                        will not see the change until restart!
  --database_connection <database>
                        Set the cell database_connection. NOTE that running
                        nodes will not see the change until restart!

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/434179

[OpenStack Infra (hudson-openstack)]

Change abandoned by Matt Riedemann (<email address hidden>) on branch: master
Review: https://review.openstack.org/434179
Reason: Already fixed here:

https://review.openstack.org/#/c/434533