wheel-index: only create index once

We saw what looks very much like the index generation racing, with
"mv" unable to close the index file ...

Move to a linear strategy so that each host finishes each task before
moving on.  Then add run_once to the index generation so only one of
the hosts in a python2/3 situation creates the index file.

Change-Id: I3761dd9d2ab752e66bcccdd2434405f11535b95f

playbooks/publish/wheel-mirror.yaml

@@ -1,5 +1,5 @@
 - hosts: all
-  strategy: free
+  strategy: linear
 
   pre_tasks:
     - name: Generate AFS slug - CentOS

roles/copy-wheels/tasks/main.yaml

@@ -3,3 +3,7 @@
 
 - name: Rebuild mirror index
   script: wheel-index.sh {{ afs_dir }}
+  # With multiple python2/3 builds, we only need one host to generate
+  # the final index.  All hosts should be finished copying under
+  # linear strategy.
+  run_once: True