Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Removed unnecessary image definitions and flavor names."
  - Removed unnecessary image definitions and flavor names.

    Change-Id: I13464d997ff9aba09a6cff7d3ff62f29df770a38
    Signed-off-by: Maciej Skrocki <maciej.skrocki@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 564496e..e91ccae 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 564496eae84e4b8916d2102222eb95877a338b20
+Subproject commit e91ccae7fa21647cd2452cdb7dbec584bad6a476