Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Merge "temp freeze releng version for stable/danube workaround" into stable/danube
  - temp freeze releng version for stable/danube workaround

    releng doesn't have a branch, so we need to freeze
    the releng version.

    There is an issue with releng fetch_os_creds.sh change

    /home/opnfv/repos/releng/utils/fetch_os_creds.sh: line 75: BRANCH:
    unbound variable

    So we need to revert back to commit before this change

    Change-Id: Ie8b0f68879a873aab584aa32b576d8b5ba602a13
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>