This is a partial revert of f12453f6cb.
The use case that change was designed to address is poorly served by
that change. The intent was to make it easier to return multiple
artifacts in multiple playbooks independently by relying on the dictionary
merge behavior of zuul_return. However, in the entirely likely case
of artifacts with generated names, it becomes difficult because Ansible
does not run jinja on dictionary keys.
Therefore, revert to the previous list behavior. A subsequent change
will add a feature to zuul_return to address the underlying issue of
returning multiple artifacts from different playbooks.
Change-Id: I0581aa68fcef320ab27c11ddd6338a15eef38ceb
e5824080d8