Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Bugfix: Error validating value 'cirros-0.3.5'

    JIRA: YARDSTICK-607

    some scenarios come with a Cirros-0.3.5 image, Yardstick's load_images.sh will
    load a second cirros-0.3.5 image, causing the following error:
    2017-03-27 07:42:58,317 yardstick.orchestrator.heat heat.py:449 INFO Creating stack 'demo-2ada98f3'
    error: failed to deploy stack: 'ERROR: Property error: : resources.ares.demo-2ada98f3.properties.image: : Error validating value 'cirros-0.3.5': No images unique match found for
    {'name': u'cirros-0.3.5'}

    This patch fixs this error by skipping load cirros image if there already exist
    one in the glance.

    Change-Id: I4d43d82d331f6575efd4560be9b6be175e03cd1b
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit 3f70095d15afb86567f885cbfbd89183d857f671)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index ffef33b..3bbb75d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit ffef33bdd91ecf1d5188e5db173fa449d4eaa741
+Subproject commit 3bbb75db603426d241cebd5425cb2b88fbd96a0c