Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - test_heat: don't waste 2 seconds in unittest

    remove duration checks and test with sleep 0

    Change-Id: I22516cde56ac3a02358ca9c3e652cead10580d76
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index eeda3f6..ab82083 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit eeda3f6b215c16cbb567168cb63d987c65f8a1b9
+Subproject commit ab820837ee980bbd356158ccfc343dee3053409a