Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Remove img_modify_playbook assignation in build_yardstick_image.yml"
  - Remove img_modify_playbook assignation in build_yardstick_image.yml

    Variable "img_modify_playbook" is being assigned in first context of
    ./ansible/build_yardstick_image.yml [1]. However this variable is never used.
    In second context [2], the variable used is a CLI input variable.

    [1]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/build_yardstick_image.yml#L187
    [2]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/build_yardstick_image.yml#L205

    JIRA: YARDSTICK-871

    Change-Id: I7bd5063c5e86520abcafdc782222ee53faf58f45
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>