Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Remove img_modify_playbook from nsb_setup.sh script

    The `img_modify_playbook` variable is not used anymore by the
    build_yardstick_image.yml. The `img_property` variable is used
    instead. The `img_modify_playbook` usage has been removed and
    the `img_property` has been introduced in
    YARDSTICK-879 JIRA story.

    JIRA: YARDSTICK-964

    Change-Id: I0239544c4f4078de7a4817292ab6bd789c21e944
    Signed-off-by: Mytnyk, Volodymyr <volodymyrx.mytnyk@intel.com>
    Signed-off-by: Alonso Hernandez, Rodolfo <rodolfo.alonso.hernandez@intel.com>