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>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 6ca8346..068cd8e 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 6ca8346532caa2997588e35c66cae86a1246e9ac
+Subproject commit 068cd8efd026f710bcc3e9e5e6892379046091ef