Allow configuring no user for sensu-api

templates/sensu-api.json.j2

@@ -1,8 +1,10 @@
 {
   "api": {
-    "host": "{{ sensu_api_host }}",
-    "port": {{ sensu_api_port }},
+    {% if sensu_api_user_name | length > 0 %}
     "user": "{{ sensu_api_user_name }}",
-    "password": "{{ sensu_api_password }}"
+    "password": "{{ sensu_api_password }}",
+    {% endif %}
+    "host": "{{ sensu_api_host }}",
+    "port": {{ sensu_api_port }}
   }
 }