The form POST operation to S3 accepts a `success_action_status` field which can be set to 200/201/204, see [here](https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html). Localstack currently handles the `success_action_redirect` field [here](https://github.com/localstack/localstack/blob/master/localstack/services/s3/s3_listener.py#L518) and so we should look into handling the `success_action_status` field too.

The 201 from AWS responds with an XML document with some fields about the object that has just been created in AWS, see below.

```
<PostResponse>
    <Location>https://s3.amazonaws.com/link-to-the-file</Location>
    <Bucket>s3-bucket</Bucket>
    <Key>development/8614bd40-691b-4668-9241-3b342c6cf429/image.jpg</Key>
    <ETag>"32-bit-tag"</ETag>
</PostResponse>
```
bucket_name = normalize_bucket_name(bucket_name)
    for bucket, notifs in S3_NOTIFICATIONS.items():
        if bucket == bucket_name:
            action = {'PUT': 'ObjectCreated', 'POST': 'ObjectCreated', 'DELETE': 'ObjectRemoved'}[method]
            
            
            if action == 'ObjectCreated' and method == 'POST':
                api_method = 'CompleteMultipartUpload'
            else:
                api_method = {'PUT': 'Put', 'POST': 'Post', 'DELETE': 'Delete'}[method]
            event_name = '%s:%s' % (action, api_method)
            for notif in notifs:
                send_notification_for_subscriber(notif, bucket_name, object_path,
                    version_id, api_method, action, event_name)
def send_notification_for_subscriber(notif, bucket_name, object_path, version_id, api_method, action, event_name):
    bucket_name = normalize_bucket_name(bucket_name)
    if (not event_type_matches(notif['Event'], action, api_method) or
            not filter_rules_match(notif.get('Filter'), object_path)):
        return
    
    message = get_event_message(
        event_name=event_name, bucket_name=bucket_name,
        file_name=urlparse.urlparse(object_path[1:]).path,
        version_id=version_id
    )
    message = json.dumps(message)
    if notif.get('Queue'):