Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Avoid pylint warning for "missing-docstring"

    Disabled pylint message for missing doctring in methods, classes and
    modules. Modules must be auto-explanatory, and some methods are trivial
    an doesn't need an explanation.

    Change-Id: I01d80737d92a7345bc6a2a6f607eb35cbf28da67
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 9613b99..a55aa99 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 9613b999df420e0f0244fd8a1b51d62a9c8de73f
+Subproject commit a55aa99ae1df1f01abd4f9a9765c136c665772bf