Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Update the dummy-scenario-heat-context testcase"
  - 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>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 5f88afc..3cc4c5f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 5f88afccf6ee9336f4e333830255f6320266a2a4
+Subproject commit 3cc4c5f8171f54232b40e393eea5873af78d302b