Use Celery instead of RQ.

Celery provides a more robust set of queueing options for both tasks and
worker processes. Updates include:
- infrastructure necessary to run Celery, including celery entrypoint
- backgrounded functions are now imported directly from atst.jobs
- update tests as-needed
- update kubernetes worker pod command

tests/utils/test_notification_sender.py

@@ -6,22 +6,19 @@ from atst.utils.notification_sender import NotificationSender
 
 
 @pytest.fixture
-def mock_queue(queue):
-    return Mock(spec=queue)
+def notification_sender():
+    return NotificationSender()
 
 
-@pytest.fixture
-def notification_sender(mock_queue):
-    return NotificationSender(mock_queue)
-
-
-def test_can_send_notification(mock_queue, notification_sender):
+def test_can_send_notification(monkeypatch, notification_sender):
+    job_mock = Mock()
+    monkeypatch.setattr("atst.jobs.send_notification_mail.delay", job_mock)
     recipient_email = "test@example.com"
     email_body = "This is a test"
 
     NotificationRecipientFactory.create(email=recipient_email)
     notification_sender.send(email_body)
 
-    mock_queue.send_notification_mail.assert_called_once_with(
+    job_mock.assert_called_once_with(
         ("test@example.com",), notification_sender.EMAIL_SUBJECT, email_body
     )