property should be required argument in unset cmd

[jiaxi]

The three command line below
openstack volume unset
openstack snapshot unset
openstack volume type unset

They should have --property as a required argument,not optional.
However the doc http://docs.openstack.org/developer/python-openstackclient/command-objects/volume-type.html

The reason is the cmd will not work without --property.
If it's optional, the cmd should work fine without it. But the truth is not.

[Steve Martinelli (stevemar)]

make them required, but keep them in the form of --property.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/203401
Committed: https://git.openstack.org/cgit/openstack/python-openstackclient/commit/?id=9c95b378323fdc37d403c3757064349c9aaa509b
Submitter: Jenkins
Branch: master

commit 9c95b378323fdc37d403c3757064349c9aaa509b
Author: jiaxi <email address hidden>
Date: Sun Jul 19 02:39:06 2015 -0400

    --property should be required in `os unset` commands

    The three commands below
      openstack volume unset
      openstack snapshot unset
      openstack volume type unset

    Should have --property as a required argument,
    not optional. The reason is the command will not work
    without --property.

    Closes-Bug: #1475872
    Change-Id: Ib9e29392472db38982cc2817af2dd5055f5a01ca