Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/euphrates'
  - Add "How to work with test case" into devguide

    JIRA: YARDSTICK-73

    add a new chapter about how to work with test case, now just add three sub chapter,

    sample test case, release test case, test case description file, will update more for

    how to write a new test case.

    Change-Id: I44906917e76ad6d4890f345c963b9e4440f6be39
    Signed-off-by: kubi <jean.gaoliang@huawei.com>
    (cherry picked from commit 1ca9d52152d54b51b4d9dcd6c429f114ad4fe24c)