Update git submodules

* Update docs/submodules/yardstick from branch 'stable/euphrates'
  - Switched nsb_setup to pull stable yardstick docker image

    Change-Id: I4474ee65899d5f60e657835e8d80de0d372abd22
    Signed-off-by: Maciej Skrocki <maciej.skrocki@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 28688e6..ddff978 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 28688e6866cea5b16870864ced9a6d4751094e6f
+Subproject commit ddff9781c46fc446380a378cbca40d9576e32b68