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/mixin.py

@@ -101,7 +101,9 @@ class UnifiedJobDeletionMixin(object):
 
 class InstanceGroupMembershipMixin(object):
     '''
-    Manages signaling celery to reload its queue configuration on Instance Group membership changes
+    This mixin overloads attach/detach so that it calls InstanceGroup.save(),
+    triggering a background recalculation of policy-based instance group
+    membership.
     '''
     def attach(self, request, *args, **kwargs):
         response = super(InstanceGroupMembershipMixin, self).attach(request, *args, **kwargs)