Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Bugfix:add storperf flavor for TC074

    JIRA: YARDSTICK-604

    the storperf container now uses a flavor called "storperf", if this flavor is
    absent, stack creating process will fail. also the openstack variables that
    passed to the storperf container also needs a update

    Change-Id: Iab4be6491cc9b24526948e5571f0476ec885a93d
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit 0691766d310e00d7f1cec763427db271076b5633)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index fd51e9f..af00409 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit fd51e9fe3f4c31be2cfd541f23c3bb5a7ba08291
+Subproject commit af004096fa17c9e29c663fc013228a9c82bd174d