Update git submodules

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>