Correct base playbook paths

We removed the implicit 'playbooks/' path.

Change-Id: I4e02a4f367fb85572203438e23572e5d8c48145e

zuul.yaml

@@ -26,8 +26,8 @@
 
       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}/