Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Implement Virtual Switch resilience test case

    This testcase tests the resilience of the Virtual
    Switch in an OpenStack environment. It can only run in a
    non Ubuntu distro because it is limited by this bug [0]

    [0]: JIRA: YARDSTICK-1140

    Change-Id: Ia1480fba9f3549d1144c80699056b501d3b07a0f
    Signed-off-by: Dimitrios Markou <mardim@intracom-telecom.com>
    (cherry picked from commit 53f5951a47fcaa5a3f8bcd6ea1020e54a1b285d3)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 8c58ec4..97d70dd 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 8c58ec4d9e2e69ced6e7773bf00c4e96b782e12f
+Subproject commit 97d70dd90f5367254fb49ce9d18e947305bc8758