Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "Update the dummy-scenario-heat-context testcase" into stable/fraser
  - Update the dummy-scenario-heat-context testcase

    The testcase is included in the docs to demonstrate definining a heat context.
    It previously used cirros, but this is not included by default.
    Set-up instructions/scripts add yardstick-image and yardstick-flavor, so these
    have been added to the testcase instead.

    A default value for context_name was also added.

    Change-Id: I5f642086971487b334ae48a5cf5c9b71e9b99e2e
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>
    (cherry picked from commit f37e4dcf771caec3be52e95047997aa22cbd7a88)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index f80cbeb..c8d64fa 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit f80cbeb50ca2254c83853fb42b95ad67d8e978c1
+Subproject commit c8d64fa99dc51f5914bcbeb01a39eee5df7b4576