Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bugfix: yardstick env grafana fails when access the server

    current timeout is 10 seconds, it would fail if grafana
    server start slow in some cases.

    JIRA: YARDSTICK-972

    Change-Id: I783206fdd1352e94ba7e498283f70b5a6834b6fa
    Signed-off-by: rexlee8776 <limingjiang@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 9e2fafb..5373748 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 9e2fafb8b86356d7c6719e593c0b086e065706c9
+Subproject commit 53737485791aef59d6178170989b54fc7139437a