Update git submodules

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)