Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add methods to get an existing stack"
  - Add methods to get an existing stack

    * Add yardstick/orchestrator/heat.py:HeatStack.get
    * Add yardstick/benchmark/contexts/heay.py:HeatContext.retrieve_existing_stack

    JIRA: YARDSTICK-886
    Change-Id: I6974b79a25f98066a49b1bc8ccd11383e7962091
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index de8ce98..9316c6c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit de8ce9889cfc9e9d62e26e53b5f27b2f4cd9ff06
+Subproject commit 9316c6c49957f2d8c680ed8acfaccac9070ed2f4