Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Added proxy setup for standard cloudimage playbooks.

    Change-Id: I4f29f584395d58b2fcece1ca8fcbda78a64e3a66
    Signed-off-by: Maciej Skrocki <maciej.skrocki@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 42e59b1..8b7cfbd 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 42e59b19264a8a31650d2a111b79d932accd8e4e
+Subproject commit 8b7cfbd3081215b2cb02e2e0db0bf11c45440f23