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

requirements/requirements.in

@@ -9,7 +9,6 @@ daphne==1.3.0   # Last before backwards-incompatible channels 2 upgrade
 decorator==4.2.1
 Django==1.11.11
 django-auth-ldap==1.2.8
-django-celery==3.2.2
 django-crum==0.7.2
 django-extensions==2.0.0
 django-jsonfield==1.0.1

requirements/requirements.txt

@@ -28,7 +28,6 @@ daphne==1.3.0
 decorator==4.2.1
 defusedxml==0.4.1         # via python-saml
 django-auth-ldap==1.2.8
-django-celery==3.2.2
 django-crum==0.7.2
 django-extensions==2.0.0
 django-jsonfield==1.0.1

requirements/requirements_dev.txt

@@ -15,7 +15,6 @@ pytest-mock
 pytest-timeout
 pytest-xdist
 logutils
-flower
 jupyter
 matplotlib
 backports.tempfile  # support in unit tests for py32+ tempfile.TemporaryDirectory