Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "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>