Update git submodules

* Update docs/submodules/yardstick from branch 'stable/euphrates'
  - Merge "Dockerfile: apt-get clean to save layer space" into stable/euphrates
  - 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>
    (cherry picked from commit 952c68f74c03facff84b987aa508040dd43ee30b)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d146325..9e5870b 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d146325720939acfbabff6da6463d15dc828c481
+Subproject commit 9e5870ba9aeed6cc7e5be92d17165130f4e121d4