Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Update virtualenv installation step in userguide"
  - Update virtualenv installation step in userguide

    Change-Id: I0b3e8f22b6878c3b2cf1a54f3c39f2888406c508
    Signed-off-by: JingLu5 <lvjing5@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 3f7829f..50a5d0c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 3f7829f0f28ce4d517ea8654122ee0d686fddf3e
+Subproject commit 50a5d0cb28d95d8edb47b1775569fcdf52dd1c96