replace celery task decorators with a kombu-based publisher

this commit implements the bulk of `awx-manage run_dispatcher`, a new
command that binds to RabbitMQ via kombu and balances messages across
a pool of workers that are similar to celeryd workers in spirit.
Specifically, this includes:

- a new decorator, `awx.main.dispatch.task`, which can be used to
  decorate functions or classes so that they can be designated as
  "Tasks"
- support for fanout/broadcast tasks (at this point in time, only
  `conf.Setting` memcached flushes use this functionality)
- support for job reaping
- support for success/failure hooks for job runs (i.e.,
  `handle_work_success` and `handle_work_error`)
- support for auto scaling worker pool that scale processes up and down
  on demand
- minimal support for RPC, such as status checks and pool recycle/reload

awx/api/views/__init__.py

@@ -3318,7 +3318,7 @@ class JobTemplateCallback(GenericAPIView):
         with transaction.atomic():
             job = job_template.create_job(**kv)
 
-        # Send a signal to celery that the job should be started.
+        # Send a signal to signify that the job should be started.
         result = job.signal_start(inventory_sources_already_updated=inventory_sources_already_updated)
         if not result:
             data = dict(msg=_('Error starting job!'))