Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Mock TRex STL libraries globally"
  - Mock TRex STL libraries globally

    TRex Python libraries are not going to be available during
    unit testing execution. Those modules calling TRex libraries
    should be mocked individually. This patch will remove the need
    of mocking those libraries per module.

    JIRA: YARDSTICK-1010

    Change-Id: I4aa11d43ecf32a3dad78f869541b4afea4ec1d28
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>