Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Improve NSB Standalone XML generation"
  - Improve NSB Standalone XML generation

    Delayed the generation of the XML file until the last step. The
    following functions will return a XML string insted:
    - Libvirt.build_vm_xml
    - SriovContext._enable_interfaces
    - OvsDpdkContext._enable_interfaces

    The XML file will be written just before copying the file to the
    compute hosting the VMs.

    JIRA: YARDSTICK-939

    Change-Id: Icc80f4741903bbe335db4ebccab395b72fa87e82
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>