Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Replace nova create instance with shade client."
  - Replace nova create instance with shade client.

    Remove redundant functions:
    *get_instances
    *get_instance_status
    *get_instance_by_name

    Function create_instance_and_wait_for_active now uses shade client.

    JIRA: YARDSTICK-1088

    Change-Id: I766da333dabbb1b83a7b4ede4bf73567ec70eb8c
    Signed-off-by: Shobhi Jain <shobhi.jain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index e937764..e6f8f28 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit e937764091fbb568e285a506582bbcc36fdb051a
+Subproject commit e6f8f286aa18d30b1b3eb09391bf8e8f6f08c672