Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Set work directory in Yardstick container"
  - Set work directory in Yardstick container

    JIRA: YARDSTICK-608

    Set the work directory to /home/opnfv/repos in yardstick docker container to
    help users better understand file structure.
    This is a improvement related with document improvement.

    Change-Id: Ief4b76e6d6898ceab4a546574f5a09d51eb0d945
    Signed-off-by: JingLu5 <lvjing5@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 6e77001..ab7cd79 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 6e77001bcd07f30f9edcf4103528002f4c4ec536
+Subproject commit ab7cd795f6d45bf8bedbe8903cae39bd447d3c19