Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Check requirements against OpenStack requirements"
  - Check requirements against OpenStack requirements

    Added a new tox environment to check the project requirements against
    the OpenStack requirements. The openstack/requirements branch used will
    depends on Yardstick release development.

    At this point, Yardstick F release must be compliant with OpenStack Pike
    release.

    Change-Id: I77a4cfa4f99674176c3f5711dc1b6787bb400ecf
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>