Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add API(v2) to get all images"
  - Add API(v2) to get all images

    JIRA: YARDSTICK-752

    API: /api/v2/yardstick/images
    METHOD: GET

    Change-Id: I7b0bba85d875575ee684cc42cad977af91d6f84e
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index be2da4d..685af77 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit be2da4d7ef13f61cfcd9b591a9b00579b896213d
+Subproject commit 685af779d6a300a4e9bac8c1aa25f0e648a25d87