Fix the inconsistent command format in openstackclient

[OpenStack Infra]

https://review.openstack.org/470201
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/python-zaqarclient" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit fa88a03b3bcb9c3b7ce03c16e9c35c5288c4f503
Author: wanghao <email address hidden>
Date: Fri Jun 2 16:32:20 2017 +0800

    Fix the inconsistent command format in openstackclient

    Change zaqarclient command to keep the same format,
    the consistent command format is 'openstack messaging XXX'.

    DocImpact

    Change-Id: I87302d016915fd7c861b0a7a542a23ae3685443e
    Closes-Bug: #1691637