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

tools/docker-compose.yml

@@ -14,10 +14,9 @@ services:
     ports:
       - "8888:8888"
       - "8080:8080"
-      - "5555:5555"
       - "8013:8013"
       - "8043:8043"
-      - "6899-6999:6899-6999"  # default port range for celery.contrib.rdb
+      - "6899-6999:6899-6999"  # default port range for sdb-listen
     links:
       - postgres
       - memcached