Correct base playbook paths
We removed the implicit 'playbooks/' path.
It also sets default timeout and nodeset values (which may be
overidden).
- pre-run: base/pre
- post-run: base/post
+ pre-run: playbooks/base/pre
+ post-run: playbooks/base/post
roles:
- zuul: openstack-infra/openstack-zuul-roles
success-url: http://logs.openstack.org/{change.number[:-2]}/{change.number}/{change.patchset}/{pipeline.name}/{buildset.uuid}/
