Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Change testcases 'cirros' image to 'yardstick-image'

    The default OpenStack 'cirros-0.3.5' image doesn't exist anymore. The
    image included by default in any testing OpenStack deployment is now
    called 'cirros-0.3.5-x86_64-disk', but this name can change.

    Because Yardstick, during the installation, deploys a customized image,
    'yardstick-image', it makes sense to use it as the default image.

    JIRA: YARDSTICK-873

    Change-Id: I8fd7cbc01d4fa52e9107f2fac7d2d9ff5d1091c7
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 3710899..b237b76 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 3710899e8909f2800a872561a1d2ffa7732dd517
+Subproject commit b237b762bbb1c9cb1a05fc46a1a67ae557791e77