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>