Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "update docker image tag to 6.1.0" into stable/fraser
  - update docker image tag to 6.1.0

    Change-Id: Ia7a0668c1c2461fceb97425bcb6ade5bdb105c79
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2568d72..59931e1 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2568d72211986a7b61decb0dd6db6233c3dc153d
+Subproject commit 59931e1276034e9304bc583e7f25518085a7ee3c