package-afs-centos: fix final copy

This is currently copying everything into

 https://tarballs.openstack.org/project-config/package-afs-centos7/x86_64/

where we don't actually want the "x86_64".  Add trailing slash so it
copies the directory contents.  Also remove any existing content with
--delete, we only want the latest.

Change-Id: I4b0fe994baaaf04db0fe552f88107d90e328f048

playbooks/package-afs-centos/post.yaml

@@ -21,6 +21,8 @@
     - name: Copy RPMs back for publication
       when: rpms_exist.stat.exists
       synchronize:
-        src: '{{ ansible_user_dir }}/rpmbuild/RPMS/x86_64'
+        src: '{{ ansible_user_dir }}/rpmbuild/RPMS/x86_64/'
         dest: '{{ zuul.executor.work_root }}/artifacts/package-afs-centos7'
         mode: pull
+        rsync_opts:
+          - --delete