Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Test case spec for SDN Virtual Switch resilience.

    This is a test case for verifying the proper functionality of
    SDN Vswitches which are located in the Compute nodes of an OpenStack
    environment.
    This test case is unaware of the configuration (non-HA,HA)

    JIRA: HA-34
    JIRA: YARDSTICK-955

    Change-Id: I701bf1f9527b1a31b9cb52e102734e10c4162766
    Signed-off-by: Dimitrios Markou <mardim@intracom-telecom.com>
    (cherry picked from commit 4cc2882c73a64e243f98c0448ca2d1f9a677953c)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 594d939..4c31d1f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 594d939576ca463e94f2906bc38b4866fd9a64eb
+Subproject commit 4c31d1fd8732e2944fb488e08fc36f331e7a8938