Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add "img_property" when building Yardstick image"
  - Add "img_property" when building Yardstick image

    Since [1], when "build_yardstick_image.yml" is executed, a variable called
    "img_property" must be set. This variable could be "normal" or "nsb",
    depending on the type of image to generate.

    [1] I9fdf30084511ca9a008df3b53887b06273bbd549

    JIRA: YARDSTICK-921

    Change-Id: Ie3ca6fcf76b94eb9604847aefa77c6a644dddd14
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>