Order post playbook content properly

These are extra ugly currently.

Change-Id: I2c7ae50fdb5e5614dd8ab2f51e8fd29516d233c9
diff --git a/zuul/cmd/migrate.py b/zuul/cmd/migrate.py
index bc2f8db..1957663 100644
--- a/zuul/cmd/migrate.py
+++ b/zuul/cmd/migrate.py
@@ -574,7 +574,9 @@
                         "Job {name} uses AFS publisher".format(name=self.name))
         if tasks:
             has_post = True
-            play = collections.OrderedDict(hosts='all', tasks=tasks)
+            play = collections.OrderedDict()
+            play['hosts'] = 'all'
+            play['tasks'] = tasks
             with open(post_playbook, 'w') as post_playbook_out:
                 ordered_dump([play], post_playbook_out)
         return has_artifacts, has_post