sent_topics can't be a list, it has to be a single regexp

Signed-off-by: Aurélien Bompard <aurelien@bompard.org>

roles/rabbit/queue/defaults/main.yml

@@ -7,4 +7,4 @@ thresholds:
   warning:  10000
   critical: 100000
 nagios_server: noc01.iad2.fedoraproject.org
-sent_topics: []
+sent_topics: null

roles/rabbit/queue/tasks/main.yml

@@ -43,12 +43,14 @@
 
 - name: Prepare the topic permissions dict
   set_fact:
-    topic_permissions: "{{ topic_permissions|default([]) + [{'vhost': vhost, 'read_priv': '.*', 'write_priv': item}] }}"
-  loop: "{{ sent_topics }}"
+    topic_permissions:
+      - vhost: vhost
+        read_priv: .*
+        write_priv: sent_topics
   tags:
     - fedora-messaging
     - rabbitmq_cluster
-  when: env == "staging"
+  when: env == "staging" and sent_topics
 
 - debug:
     msg: "Topic permissions: {{ topic_permissions|default([]) }}"