Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - NSB: update vfw scale-out testcases

    Change-Id: I6bd8a87f70954f288975e3e73e3cfbd36ae561dc
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 9b4e18d..39ead7a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 9b4e18d1069c3d86af18be5adacde93fa2626bcc
+Subproject commit 39ead7aee3610dddbdeae33a5776ae5b98dfabea