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/main/scheduler/tasks.py

@@ -2,30 +2,24 @@
 # Python
 import logging
 
-# Celery
-from celery import shared_task
-
 # AWX
 from awx.main.scheduler import TaskManager
+from awx.main.dispatch.publish import task
 
 logger = logging.getLogger('awx.main.scheduler')
 
-# TODO: move logic to UnifiedJob model and use bind=True feature of celery.
-# Would we need the request loop then? I think so. Even if we get the in-memory
-# updated model, the call to schedule() may get stale data.
 
-
-@shared_task()
+@task()
 def run_job_launch(job_id):
     TaskManager().schedule()
 
 
-@shared_task()
+@task()
 def run_job_complete(job_id):
     TaskManager().schedule()
 
 
-@shared_task()
+@task()
 def run_task_manager():
     logger.debug("Running Tower task manager.")
     TaskManager().schedule()