indexer: Task must inherit from swh.scheduler.tasks

We must inherit from the tasks defined in the scheduler:

1. to centralize the celery configuration there (well in best effort mode for now)
1. to work ;)

[2018-10-25 17:32:54,017: ERROR/MainProcess] Task swh.indexer.tasks.OrchestratorAllContents[b39b8a5b-a046-4e0e-98b0-88aeab65af5a] raised unexpected: NotImplementedError('Tasks must define the run method.',)
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/celery/app/task.py", line 437, in run
    raise NotImplementedError('Tasks must define the run method.')
NotImplementedError: Tasks must define the run method.

dashboard: http://kibana0.internal.softwareheritage.org:5601/app/kibana#/dashboard/289ce780-d88d-11e8-b8ce-cf95f437ce37

Migrated from T1294 (view on Phabricator)