Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Make GenericTrafficGen class abstract

    GenericTrafficGen class is now an abstract class. Only optional methods
    are implemented.
      - 'run_traffic' and 'terminate' are mandatory.
      - 'listen_traffic', 'verify_traffic' and 'wait_for_instance' are
        optional. By default these methods doesn't execute any action.

    JIRA: YARDSTICK-853

    Change-Id: I2befdaa337af79cc2364bdd7c66183c31c5ab69a
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>