Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Yardstick virtualenv support

    JIRA: YARDSTICK-620

    In the patch, I separate python-setuptools from install.sh
    and separate appdirs from requirements.txt because there will be
    error when install them in virtualenv.
    Also I will update the documentation when this patch merged;

    Change-Id: I99a532d1851983775869e7c5e8e65fd0229e86df
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>
    (cherry picked from commit bb365d411d88a71871993b4ed803c2ffbbcb79bf)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 87d8de2..7d08b88 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 87d8de27074f71db307891e1230a9341f852d945
+Subproject commit 7d08b884344651d9ba3dd2830dcd976d2adba9f5