Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Replace assertEqual(None, xx) with assertIsNone(xx)"
  - Replace assertEqual(None, xx) with assertIsNone(xx)

    Change-Id: Ie956d8064a8fbcbd3d56c5a79c4c613d35184af4
    JIRA: YARDSTICK-942
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index c27266f..a74ad5a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit c27266f509dd1fb07d7b03e5c6befe489a95a423
+Subproject commit a74ad5a1ec1a73389c5983440b2031b0bc72cea1