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

tools/docker-compose/haproxy.cfg

@@ -22,11 +22,6 @@ frontend localnodes_ssl
     mode tcp
     default_backend nodes_ssl
 
-frontend flower
-    bind *:5555
-    mode http
-    default_backend flower_nodes
-
 frontend rabbitctl
     bind *:15672
     mode http
@@ -51,18 +46,6 @@ backend nodes_ssl
     server awx_2 awx_2:8043
     server awx_3 awx_3:8043
 
-backend flower_nodes
-    mode http
-    balance roundrobin
-    option forwardfor
-    option http-pretend-keepalive
-    http-request set-header X-Forwarded-Port %[dst_port]
-    http-request add-header X-Forwarded-Proto https if { ssl_fc }
-    #option httpchk HEAD / HTTP/1.1\r\nHost:localhost
-    server awx_1 awx_1:5555
-    server awx_2 awx_2:5555
-    server awx_3 awx_3:5555
-
 backend rabbitctl_nodes
     mode http
     balance roundrobin