Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "[docs] Add conventions for writing unit tests."
  - [docs] Add conventions for writing unit tests.

    Added details of conventions used when writing unit tests.

    JIRA: YARDSTICK-1335
    Change-Id: I7bb141126968c88c8a2d7e0dce4f33ed6f79cee4
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 78b5a19..f64419c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 78b5a19e3071cbc080eeae0bdf22f5ab80c03180
+Subproject commit f64419cc7d16a679ed130dfe7592533b3cf9fdc1