Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Modify IxNextgen API for traffic model configuration

    Changes to IxNextgen API:
     - modify API for traffic model configuration so that it can be used
       to create/configure different models
     - add methods to start and stop protocols on IxNetwork
     - add method for getting assigned ports

    JIRA: YARDSTICK-1455

    Change-Id: I43c10fea9ba83994caf373ceff8b0252a45439b3
    Signed-off-by: Serhiy Pshyk <serhiyx.pshyk@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index ee5d641..7237241 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit ee5d6413dbcbab40234e47ed3e74b85b704d6493
+Subproject commit 7237241a396871d637f9906c1fc93a8587a7b726