Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - 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 6e3cb93..abc3769 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 6e3cb9346cb11acd6bf0882bd7f715b879246909
+Subproject commit abc37694c6b0d1d80fa5942386d41f69d1bc085a