Add python-openstackclient in docker image

Add python-openstackclient in docker image

"openstack: command not found" error occurred in [1].
The version of python-openstackclient is listed in [2].
This program should be installed in the container image
to make CI work for now.

[1]: https://build.opnfv.org/ci/job/yardstick-daisy-baremetal-daily-master/114/
[2]: https://github.com/openstack/requirements/blob/stable/pike/global-requirements.txt

Change-Id: Idb50e26bb2ca36f2dd6b6dddf94680d479cb9c9f
Signed-off-by: Alex Yang <yangyang1@zte.com.cn>