Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - fix multi-monitor unitest hang

    need to mock SSH.from_node().execute.return_value
    because we switch most SSH calls to use from_node()
    factory function

    Change-Id: Ibea525c2fcc243d41de8ccbcd40bc98532d69006
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
    (cherry picked from commit 5271e612d427b29c2da84f8417f4192427ae3977)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index fc1b6f9..ad3ff4c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit fc1b6f9e2129fabfebbe0e6ea210f48edc0ff27b
+Subproject commit ad3ff4cbaf92808801c9157d25416da6cff82666