Collect output from a sphinx build as a tarball

By default, this copies the output from the sphinx build on the worker
to the log root of the executor as a tarball, and then extracts the
archive into the log root for viewing.

**Role Variables**

.. zuul:rolevar:: sphinx_build_dir
   :default: doc/build

   Directory relative to zuul_work_dir where build output should be
   found.

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

   The location of the main working directory of the job.

.. zuul:rolevar:: sphinx_pdf_files
   :default: list

   A list of file names of PDF files to collect.
   By default, the list contains as entry only
   ``doc-{{ zuul.project.short_name }}.pdf``.

.. 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.