confusion help msg for nova volume-update

[Liyingjun]

$ nova help volume-update
usage: nova volume-update <server> <volume> <volume>

Update volume attachment.

Positional arguments:
  <server> Name or ID of server.
  <volume> Attachment ID of the volume.
  <volume> ID of the volume to attach.

There are two same arguments `<volume>` in the usage message, which is confused for the user. It's better to change the first <volume> to another name.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/98017

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/98017
Committed: https://git.openstack.org/cgit/openstack/python-novaclient/commit/?id=54040875e036679c330548c46946f31a248ae6f4
Submitter: Jenkins
Branch: master

commit 54040875e036679c330548c46946f31a248ae6f4
Author: liyingjun <email address hidden>
Date: Wed Jun 4 07:26:37 2014 +0800

    Change help message for volume-update

    `nova volume-update <server> <volume> <volume>` to
    `nova volume-update <server> <attachment> <volume>`

    Change-Id: Ie820843c3e8d23659075c1d6b4fdd47f639b3b58
    Closes-bug: 1326613