Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bumped samplevnfs flavor disk size to 6G.

    Since we increased the images size the 4G is not sufficient anymore.

    Change-Id: Iae25cf4cfb7a6cc69c8d28771c183a2342ac38d0
    Signed-off-by: Maciej Skrocki <maciej.skrocki@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 5525e55..1a4eb9c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 5525e551cfecc9ac653778f3f498502b6922f2a8
+Subproject commit 1a4eb9cbdab6010e6c3f8f9842b856513aba2232