Add letter tasks to manifest and queue name to config test

tests/app/test_config.py

@@ -63,7 +63,7 @@ def test_cloudfoundry_config_has_different_defaults():
 def test_queue_names_all_queues_correct():
     # Need to ensure that all_queues() only returns queue names used in API
     queues = QueueNames.all_queues()
-    assert len(queues) == 12
+    assert len(queues) == 13
     assert set([
         QueueNames.PRIORITY,
         QueueNames.PERIODIC,
@@ -76,5 +76,6 @@ def test_queue_names_all_queues_correct():
         QueueNames.RETRY,
         QueueNames.NOTIFY,
         QueueNames.CREATE_LETTERS_PDF,
-        QueueNames.CALLBACKS
+        QueueNames.CALLBACKS,
+        QueueNames.LETTERS,
     ]) == set(queues)