Add comment to top of zuul.yaml pointing to other files
Add comment to top of zuul.yaml pointing to other files

Zuul v3 config is spread across a bunch of repos, so figuring out where
somethig is defined may be daunting for a newcommer. Add a comment
to the top of the file pointing people at other repos they should be
aware of.

Also remove self-referencing roles declaration since job repos
automatically get their own roles now.

Change-Id: Ib9f966ff96f6617e8a7ce3b1e9aa4862f9e12a75

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs