Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Dockerfile: add ping for prepare_env.sh

    INFO: Creating openstack credentials ..
    Verifying connectivity to 10.20.0.2...
    /home/opnfv/repos/yardstick//tests/ci/prepare_env.sh: line 56: ping: command not found
    /home/opnfv/repos/yardstick//tests/ci/prepare_env.sh: line 56: ping: command not found

    https://build.opnfv.org/ci/view/yardstick/job/yardstick-fuel-baremetal-daily-master/2400/console

    JIRA: YARDSTICK-1201

    Change-Id: Ib9e139046724ece4be7a85cc1ae74cbb0ec8e203
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 256fc55..2568d72 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 256fc55e7fabfc70a445d2be8fd879fbfe218eb2
+Subproject commit 2568d72211986a7b61decb0dd6db6233c3dc153d