Add support for roles in zuul

This adds support for Ansible roles in Zuul-managed repos.  It
is currently limited to repos within the same source, which is
something we should fix.

We also plan to add support for roles from Ansible Galaxy in a
future change.

Change-Id: I7af4dc1333db0dcb9d4a8318a4a95b9564cd1dd8
10 files changed