Generate stackviz report.

Generate stackviz report using subunit and dstat data, using
the stackviz archive embedded in test images.

**Role Variables**

.. zuul:rolevar:: devstack_base_dir
   :default: /opt/stack

   The devstack base directory.

.. zuul:rolevar:: stage_dir
   :default: /opt/stack/logs

   The stage directory where the input data can be found and
   the output will be produced.

.. zuul:rolevar:: test_results_stage_name
   :default: test_results

   The name of the subunit file to be used as input.