Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "nsb: update HEAT testcases"
  - nsb: update HEAT testcases

    use yardstick-samplevnfs image name
    always create DPDK flavor, we require 1 socket, 10 cores
    remove hardcoded external network
    add gateway_ip: null to prevent multiple default gateways ssh bug with Heat
    add enable_dhcp: false

    Change-Id: I85787b72bf6ebf1c330f20f50f7a7a2b12aa995e
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>