Stop binding tasks to a specific instance of the celery app

The celery.shared_task decorator allows late-binding of tasks to any celery app,
which is well suited for our "task plugin" architecture.

swh/lister/gnu/tasks.py

@@ -2,17 +2,17 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from swh.scheduler.celery_backend.config import app
+from celery import shared_task
 
 from .lister import GNULister
 
 
-@app.task(name=__name__ + '.GNUListerTask')
+@shared_task(name=__name__ + '.GNUListerTask')
 def list_gnu_full(**lister_args):
     'List lister for the GNU source code archive'
     GNULister(**lister_args).run()
 
 
-@app.task(name=__name__ + '.ping')
+@shared_task(name=__name__ + '.ping')
 def _ping():
     return 'OK'