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

installer/roles/image_build/templates/Dockerfile.j2

@@ -41,6 +41,7 @@ RUN yum -y install epel-release && \
     rm -rf /root/.cache
 
 RUN mkdir -p /var/log/tower
+RUN chmod -R g+w /var/log/tower
 RUN mkdir -p /etc/tower
 COPY {{ awx_sdist_file }} /tmp/{{ awx_sdist_file }}
 RUN OFFICIAL=yes pip install /tmp/{{ awx_sdist_file }}