Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - pip install yardstick failed in Apex

    JIRA: YARDSTICK-625

    In Apex, yardstick uninstall and install via pip failed.
    This bug blocks yardstick CI jobs.
    This patch enables pip install yardstick in editable mode.
    So the install_yardstick() in yardstick_verify is unnecessary.

    Change-Id: If290dac5fee5584601064ae9753ae2340643d3ca
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit d2cb7ce7331d8da4c3b5b70b09dac49cf65d1710)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 9c7086c..77f471b 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 9c7086ce2f23aa4dd47373208651ff1e560c41de
+Subproject commit 77f471be8bd681c7a4d41d0549104e231c437e95