Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Bugfix: take test case modification into effect in load_images.sh

    JIRA: YARDSTICK-622

    Bug log: https://build.opnfv.org/ci/view/armband/job/yardstick-fuel-armband-baremetal-daily-danube/24/console

    This error is because the file changes done in the load_images.sh is not updated.
    After file modification, pip install -U . is required to tack changes into effect.

    Change-Id: I8be3ffc963e3d5cec861b68a33efc2fa5bdabbd2
    Signed-off-by: JingLu5 <lvjing5@huawei.com>