Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Improve "Libvirt.create_snapshot_qemu" function"
  - Improve "Libvirt.create_snapshot_qemu" function

    Improved "Libvirt.create_snapshot_qemu" function:
    - Check if the base image is present in the remote host.
      If not, try to copy from the execution host.
    - Check the execution status. In case the command fails, raise an
      exception.

    JIRA: YARDSTICK-944

    Change-Id: I78bd0d3ed6a1f35ed772c0d192bb240009a580ed
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>