Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "dump CI failures with kpartx and no /dev/loop devices"
  - dump CI failures with kpartx and no /dev/loop devices

    CI is failing due to unable to find loop device for kpartx

    "cmd": [
        "kpartx",
        "-l",
        "/tmp/workspace/yardstick/yardstick-xenial-server.raw"
    ]

    "stderr": "mount: could not find any device /dev/loop#Bad address\ncan't set up loop",

    This error occurs when kpartx can't find any loop devices to use

    https://build.opnfv.org/ci/job/yardstick-compass-virtual-daily-master/3261/console

    JIRA: YARDSTICK-1054
    JIRA: YARDSTICK-1096

    Change-Id: Ib6131ce29c4f9e81386eb5471dd6107825798620
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a6adb45..9875504 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a6adb45bb0ee22f2548004afe0d8a7b09e7c15c4
+Subproject commit 9875504ba3aead6bdabd12adb1268d0a44f387dd