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/settings/local_settings.py.example

@@ -39,13 +39,13 @@ if is_testing(sys.argv):
             'ENGINE': 'django.db.backends.sqlite3',
             'NAME': os.path.join(BASE_DIR, 'awx.sqlite3'),
             'TEST': {
-                # Test database cannot be :memory: for celery/inventory tests.
+                # Test database cannot be :memory: for tests.
                 'NAME': os.path.join(BASE_DIR, 'awx_test.sqlite3'),
             },
         }
     }
     
-# Celery AMQP configuration.
+# AMQP configuration.
 BROKER_URL = 'amqp://guest:guest@localhost:5672'
 
 # Set True to enable additional logging from the job_event_callback plugin
@@ -94,8 +94,7 @@ REMOTE_HOST_HEADERS = ['REMOTE_ADDR', 'REMOTE_HOST']
 # REMOTE_HOST_HEADERS will be trusted unconditionally')
 PROXY_IP_WHITELIST = []
 
-# Define additional environment variables to be passed to subprocess started by
-# the celery task.
+# Define additional environment variables to be passed to ansible subprocesses
 #AWX_TASK_ENV['FOO'] = 'BAR'
 
 # If set, use -vvv for project updates instead of -v for more output.