Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Improve Yardstick user-guide"
  - Improve Yardstick user-guide

    This patch adds a short description on how to use the "yardstick env prepare"
    command in a Non-OPNFV installer environtment.

    Change-Id: Idf54bf5318499d2b129a1445e0758e351945e755
    Signed-off-by: JingLu5 <lvjing5@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 674a990..cd5bb5f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 674a9902d40aef9a4c80669a0d00fc47447a806e
+Subproject commit cd5bb5f2dd2ec89e6c814c2771402aa8e39e10ef