Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Correctly set PYTHONPATH in Dockerfile

    JIRA: YARDSTICK-1144

    Change-Id: I8c0fe229ed620417a85356e017b171260373c43f
    Signed-off-by: John O Loughlin <john.oloughlin@intel.com>
    (cherry picked from commit a114dc39f003beb1bb1ed12ed17fd3a6d0e7deaa)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index c87a543..578049f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit c87a5438961cba359629800b31df3ad7f656aff8
+Subproject commit 578049f6e4441d623b4345489204ad781465227e