Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/euphrates'
  - Merge "Refactor userguide "Yardstick Installation"" into stable/euphrates
  - Refactor userguide "Yardstick Installation"

    [1] must be refactored in order to:
      * Include proxy options.
      * Use OpenStack client, instead of project CLI.
      * Use sudo in some commands.
      * Adapt the document to rst coding style.
      * Fix minor errors.

    JIRA: YARDSTICK-828

    [1] https://github.com/opnfv/yardstick/blob/923aa1738cbf5a8053e77e00937fc26f2142f6a3/docs/testing/user/userguide/04-installation.rst

    Change-Id: Ice63f344a8a87221bdf4daa6153b9ccac321e5b7
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
    (cherry picked from commit d20b876a02904653cf390a97c2ed9973821dc508)