Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Bugfix: remove double quote around SECURE variable

    JIRA: YARDSTICK-689

    This patch remove the double quote around the SECURE varibale as it may cause the fowllowing error:
    $ openstack "${SECURE}" image list
    openstack: ' image list' is not an openstack command. See 'openstack --help'.
    Did you mean one of these?
      access token create
      address scope create
      address scope delete
      address scope list
      address scope set
      address scope show
      aggregate add host
      aggregate create
      ...

    Change-Id: Ibb22e85b3dd89b0e3b62821bd5ebe4f155886ffc
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit b04755b5148daf36db436dc066e1bef2c93f1879)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index abc3769..2f9126f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit abc37694c6b0d1d80fa5942386d41f69d1bc085a
+Subproject commit 2f9126f350bff38eec2ff452866e5a2ca3d0bdd7