Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Dockerfile: apt-get clean to save layer space"
  - Dockerfile: apt-get clean to save layer space

    it seems like normally Dockerfiles do an apt-get clean
    to remove apt cache and save space on docker layers

    Change-Id: I2ac9d30daa917973c3ba76a50cda67d6ecb8c47a
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 3636829..31b02e5 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 3636829d1fd2053d09c95a98ec1a697c601e0e6f
+Subproject commit 31b02e5b95d8b4076ef60667d051a8a1a6b072ba