Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Fix Cirros image name in CI script"
  - Fix Cirros image name in CI script

    Fixed the name of Cirros image used in OpenStack. The image name
    is written in lowercase letters.

    JIRA: YARDSTICK-842

    Change-Id: Iff0b494125be61a055da753eaf57903b64f18509
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a75bc15..d2c7cc4 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a75bc1531caa213cdd0a335b2b3332f844c88ef1
+Subproject commit d2c7cc4e9768ed003257a95c92cdb278d516761b