Only copy openafs centos rpms if we built them

The openafs centos packaging job only builds new rpms if the desired
version doesn't have an rpm published. Otherwise it short circuits and
the job ends. Unfortunately the post tasks assume the rpm will always be
built so this job then fails. Fix this by checking for rpms before
running the post tasks.

Change-Id: I132b82d437a19f855a49d175a912792417a85ae9

playbooks/package-afs-centos/post.yaml

@@ -1,13 +1,24 @@
 - hosts: all
   tasks:
 
+    # We only build an RPM if one doesn't exist for the desired version.
+    - name: Check for built RPMs
+      stat:
+        path: '{{ ansible_user_dir }}/rpmbuild/RPMS/x86_64'
+        get_checksum: false
+        get_mime: false
+        get_md5: false
+      register: rpms_exist
+
     - name: Ensure artifacts directory exists
+      when: rpms_exist.stat.exists
       file:
         state: directory
         path: '{{ zuul.executor.work_root }}/artifacts/package-afs-centos7'
       delegate_to: localhost
 
     - name: Copy RPMs back for publication
+      when: rpms_exist.stat.exists
       synchronize:
         src: '{{ ansible_user_dir }}/rpmbuild/RPMS/x86_64'
         dest: '{{ zuul.executor.work_root }}/artifacts/package-afs-centos7'