[ansible-lint] prefix variable names for rabbit/user role

ansible-lint requires that variables for roles are prefixed with the
name of the role. This commit prefixes the variables for the
rabbit/user role with user_ as required by ansible-lint

Signed-off-by: Ryan Lerch <rlerch@redhat.com>

roles/messaging/base/tasks/main.yml

@@ -56,8 +56,8 @@
 - name: "make sure the user exists on broker"
   include_role: name=rabbit/user
   vars:
-  - username: "{{ item.username }}{{ env_suffix }}"
-    sent_topics: "{{ item.sent_topics }}"
+  - user_username: "{{ item.username }}{{ env_suffix }}"
+    user_sent_topics: "{{ item.sent_topics }}"
   with_items: "{{ messaging.certificates }}"
   tags:
   - fedora-messaging