Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Jörgen Karlsson standing down as Yardstick committer

    https://lists.opnfv.org/pipermail/opnfv-tsc/2018-May/004333.html

    Change-Id: Ida2692c9f5f8e41580a653c8e9b5a05b5d99df00
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index beca893..d2c31b1 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit beca893904ff893d2fc944eb475d045cbe22c39b
+Subproject commit d2c31b133a1429f3028d7f4c2969c1c5c52903a6