Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Remove tool provisioning in PROX helper

    Tool provisioning in PROX setup environment helper is not needed [1]. The
    tool (PROX traffic injector) is already provided during the VNF building:
      ./ansible/nsb_setup.yml -->
      ./ansible/build_yardstick_image.yml -->
      ./ansible/ubuntu_server_cloudimg_modify_samplevnfs.yml -->
      ./ansible/roles/install_samplevnf

    [1]https://github.com/opnfv/yardstick/blob/master/yardstick/network_services/vnf_generic/vnf/prox_helpers.py#L641

    JIRA: YARDSTICK-872

    Change-Id: I0f925a7967a35a97901fbe5053793a791a7b1b01
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>