Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Update the pinned sampleVNF version

    JIRA: YARDSTICK-1414

    Updated the sampleVNF version to the most recent validated commit.
    Provided comments on accepted version types and values, and on where
    users can find alternative values, in case they want to try something
    different than the version recommended and validated for Yardstick/NSB.

    Change-Id: I336f96237fcfa8b580c91c787d722d368cc6c015
    Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>