Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Move test requirements into test-requirements.txt"
  - Move test requirements into test-requirements.txt

    Move the packages required for testing into test-requirements.txt
    This follows conventions used in OpenStack projects.

    JIRA: YARDSTICK-837
    Change-Id: If08fc416aa21b11250066bc19a54daa35e863407
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index ac33f63..40252f6 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit ac33f63b6f7f61de1e2dedcd65b18a056cd8aa81
+Subproject commit 40252f6eea7a5dc622ce05bcac7762ac3b5e0a49