Collect subunit outputs

**Role Variables**

.. zuul:rolevar:: zuul_work_dir
   :default: {{ ansible_user_dir }}/{{ zuul.project.src_dir }}

   Directory to work in. It has to be a fully qualified path.

.. zuul:rolevar:: fetch_subunit_output_additional_dirs
   :default: []

   List of additional directories which contains subunit files
   to collect. The content of zuul_work_dir is always checked,
   so it should not be added here.

.. zuul:rolevar:: zuul_use_fetch_output
   :default: false

   Whether to synchronize files to the executor work dir, or to copy them
   on the test instance.
   When set to false, the role synchronizes the file to the executor.
   When set to true, the job needs to use the fetch-output role later.