Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add container ping VM test case"
  - Add container ping VM test case

    JIRA: YARDSTICK-769

    This is demand from openretriever project(https://jira.opnfv.org/browse/CONTAINER-19).
    Since we have the kubernetes context now and we have first container test case.
    We can have a new test case that make container(k8s) ping VM(openstack).

    Change-Id: Ia1724130d92d447836529b370510fe1bf1738723
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index bdcc2f1..dfac81a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit bdcc2f13601ef1a2134559fd7d67562dbe8d360b
+Subproject commit dfac81ab72dbed41b79ab49c3afd9d6e41ffe43f