Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Merge "improve git code with devstack functions" into stable/danube
  - improve git code with devstack functions

    if we aren't re-cloning we still need to make sure
    we update the branch/tag/commit to the version from the remote

    so we need to fetch and then checkout again

    it is more complicated.  Dockerfile does a shallow
    clone, so we don't get all the remote ref information.

    To clone a random remote commit id we have to unshallow

    Change-Id: If15f504b6ff2cfdfc8894fd3b37e687a19616714
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>