Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bugfix: load_image will move image file if image already exist in /home/opnfv/images

    JIRA: YARDSTICK-678

    Currently if yardstick-image exist in /home/opnfv/images, load_image.sh
    script will not build image, but this script will remove it. We should not
    remove the image file which user offer.

    Change-Id: I74ff6717606ff1dd9eacde4142e62d3923394baa
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 6746ada..0509cf3 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 6746ada2e8c837da948c3260ceb8829ea1fc27de
+Subproject commit 0509cf3e141b550653e642083907a584450a7fdf