Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add API(v1) to get real time log"
  - Add API(v1) to get real time log

    JIRA: YARDSTICK-805

    We need to get real time log if we use API(v1) run test case.

    API: /yardstick/tasks/<task_id>/log
    For example:
    http://ip:port/yardstick/tasks/14795be8-f144-4f54-81ce-43f4e3eab33f/log?index=0

    Change-Id: Ic0bf10f26f4f07d3264f2415c4923a1bfded2f81
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 3d462aa..39f20c5 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 3d462aaa3d87a2298cf1f9a859e996d708f917fd
+Subproject commit 39f20c597a8a602016f5200a2e60a2ef412d8b87