Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/gambia'
  - Merge changes I526c328f,I7bb14112,Idfe77900 into stable/gambia

    * changes:
      [docs] Update on Yardstick installation
      [docs] Add conventions for writing unit tests.
      [docs] Add developer guidelines for contribution

  - [docs] Update on Yardstick installation

    Added details how to install Yardstick using install.yaml.

    Updated:
     - 04-installation.rst
     - 13-nsb-installation.rst

    JIRA: YARDSTICK-1335

    Change-Id: I526c328f1b197833a2cf496cfef15ba89c11fab1
    Signed-off-by: Stepan Andrushko <stepanx.andrushko@intel.com>
  - [docs] Add conventions for writing unit tests.

    Added details of conventions used when writing unit tests.

    JIRA: YARDSTICK-1335
    Change-Id: I7bb141126968c88c8a2d7e0dce4f33ed6f79cee4
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>
  - [docs] Add developer guidelines for contribution

    A new section was added to the devguide, stating
    expectations for code contributions.

    * What to include in a change
    * Code style guidelines
    * How to run tests

    JIRA: YARDSTICK-1335
    Change-Id: Idfe7790042a3a3fe6cd8763374b10f2268bd63cd
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>