Update/handle schedule activity not exist

The purpose of these changes is:

- To schedule an activity without getting its type from SWF with an API call. Formerly it used the `.get_or_create()` method of a `QuerySet` to get the type and create it if necessary. It involved one API call for each task to schedule.
- To launch the tests without AWS credentials and consequently fix the Travis build!
@ggreg `swf` is not imported, right? Only `swf.model` is imported.