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/models/ha.py

@@ -32,7 +32,7 @@ __all__ = ('Instance', 'InstanceGroup', 'JobOrigin', 'TowerScheduleState',)
 
 
 def validate_queuename(v):
-    # celery and kombu don't play nice with unicode in queue names
+    # kombu doesn't play nice with unicode in queue names
     if v:
         try:
             '{}'.format(v.decode('utf-8'))