listers: Allow to override policy and priority for scheduled tasks

Prior to this commit, the policy and priority were hard-coded.
The default values are now the old hard-coded values.

This will allow to develop a cli to trigger forges listing with oneshot policy
and some priority tasks. Thus ingesting those faster and without manual
interventation as we currently do.

swh/lister/cran/lister.py

@@ -27,7 +27,8 @@ class CRANLister(SimpleLister):
         needed for the ingestion task creation.
         """
         return create_task_dict(
-            'load-%s' % origin_type, 'recurring',
+            'load-%s' % origin_type,
+            kwargs.get('policy', 'recurring'),
             kwargs.get('name'), origin_url, kwargs.get('version'),
             project_metadata=self.descriptions[kwargs.get('name')])