Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/gambia'
  - Merge "[docs] Test case documentation for vEPC testing" into stable/gambia
  - [docs] Test case documentation for vEPC testing

    JIRA: YARDSTICK-1349

    The test case document will cover the following:
    - test purpose
    - test configuration
    - pre-test conditions
    - test sequence
    - test verdict

    Change-Id: I2841a7ff3d248bb3e4739579fad722dc02b5504f
    Signed-off-by: Abhijit Sinha <abhijit.sinha@intel.com>
    (cherry picked from commit c8bde3b59031b352bf09302ae7644436860602d7)