Put service callback retries on a different queue

At the moment, if a service callback fails, it will get put on the retry queue.
This causes a potential problem though:

If a service's callback server goes down, we may generate a lot of retries and
this may then put a lot of items on the retry queue. The retry queue is also
responsible for other important parts of Notify such as retrying message
delivery and we don't want a service's callback server going down to have an
impact on the rest of Notify.

Putting the retries on a different queue means that tasks get processed
faster than if they were put back on the same 'service-callbacks' queue.

tests/app/celery/test_service_callback_tasks.py

@@ -109,7 +109,7 @@ def test__send_data_to_service_callback_api_retries_if_request_returns_500_with_
         send_delivery_status_to_service(notification.id, encrypted_status_update=encrypted_data)
 
     assert mocked.call_count == 1
-    assert mocked.call_args[1]['queue'] == 'retry-tasks'
+    assert mocked.call_args[1]['queue'] == 'service-callbacks-retry'
 
 
 @pytest.mark.parametrize("notification_type",

tests/app/test_config.py

@@ -60,7 +60,7 @@ def test_load_config_if_cloudfoundry_not_available(reload_config):
 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) == 17
+    assert len(queues) == 18
     assert set([
         QueueNames.PRIORITY,
         QueueNames.PERIODIC,
@@ -74,6 +74,7 @@ def test_queue_names_all_queues_correct():
         QueueNames.NOTIFY,
         QueueNames.CREATE_LETTERS_PDF,
         QueueNames.CALLBACKS,
+        QueueNames.CALLBACKS_RETRY,
         QueueNames.LETTERS,
         QueueNames.SMS_CALLBACKS,
         QueueNames.SAVE_API_EMAIL,