Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - ansible: fix corrupt /etc/fstab boot error

    Change-Id: I71fdef5deff75cad434bf19f0a7e507c1dcc4ed3
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a691427..2251057 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a691427480c3a8cad60b275d47d7ddbde1c26f64
+Subproject commit 2251057b0222464398ae86d92b5904f418af6ed8