Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Make "Scenario" class abstract"
  - Make "Scenario" class abstract

    All scenario child classes must implement "run" method.

    JIRA: YARDSTICK-1065

    Change-Id: I35b78e380620967b49cd8cd23777a1aee6dfd140
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 9875504..94431ba 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 9875504ba3aead6bdabd12adb1268d0a44f387dd
+Subproject commit 94431babb856f079812aa518ee562f763057263a