Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Replace glance delete image with shade client.

    Remove get_image_id function.
    Function delete_image now uses shade client.

    JIRA: YARDSTICK-892

    Change-Id: I6e8510dfa49aa14786ed7ac3382b85c4e699fb9e
    Signed-off-by: Shobhi Jain <shobhi.jain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2e6ac12..d6c1b8b 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2e6ac1229309227398cb651e68ec062dbb95c955
+Subproject commit d6c1b8b7bfd257efd5fe2df30dd450bdb2c57c7d