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

INSTALL.md

@@ -236,7 +236,7 @@ Using /etc/ansible/ansible.cfg as config file
 }
 Operations to perform:
   Synchronize unmigrated apps: solo, api, staticfiles, messages, channels, django_extensions, ui, rest_framework, polymorphic
-  Apply all migrations: sso, taggit, sessions, djcelery, sites, kombu_transport_django, social_auth, contenttypes, auth, conf, main
+  Apply all migrations: sso, taggit, sessions, sites, kombu_transport_django, social_auth, contenttypes, auth, conf, main
 Synchronizing apps without migrations:
   Creating tables...
     Running deferred SQL...
@@ -548,7 +548,7 @@ Using /etc/ansible/ansible.cfg as config file
 }
 Operations to perform:
   Synchronize unmigrated apps: solo, api, staticfiles, messages, channels, django_extensions, ui, rest_framework, polymorphic
-  Apply all migrations: sso, taggit, sessions, djcelery, sites, kombu_transport_django, social_auth, contenttypes, auth, conf, main
+  Apply all migrations: sso, taggit, sessions, sites, kombu_transport_django, social_auth, contenttypes, auth, conf, main
 Synchronizing apps without migrations:
   Creating tables...
     Running deferred SQL...