generalize stdout event processing to emit events for *all* job types

* introduces three new models: `ProjectUpdateEvent`,
  `InventoryUpdateEvent`, and `SystemJobEvent`
* simplifies the stdout callback management in `tasks.py` - now _all_
  job run types capture and emit events to the callback receiver
* supports stdout reconstruction from events for stdout downloads for
  _all_ job types
* configures `ProjectUpdate` runs to configure the awx display callback
  (so we can capture real playbook events for `project_update.yml`)
* ProjectUpdate, InventoryUpdate, and SystemJob runs no longer write
  text blobs to the deprecated `main_unifiedjob.result_stdout_text` column

see: https://github.com/ansible/awx/issues/200

awx/lib/awx_display_callback/events.py

@@ -123,6 +123,8 @@ class EventContext(object):
             event_data['job_id'] = int(os.getenv('JOB_ID', '0'))
         if os.getenv('AD_HOC_COMMAND_ID', ''):
             event_data['ad_hoc_command_id'] = int(os.getenv('AD_HOC_COMMAND_ID', '0'))
+        if os.getenv('PROJECT_UPDATE_ID', ''):
+            event_data['project_update_id'] = int(os.getenv('PROJECT_UPDATE_ID', '0'))
         event_data.setdefault('pid', os.getpid())
         event_data.setdefault('uuid', str(uuid.uuid4()))
         event_data.setdefault('created', datetime.datetime.utcnow().isoformat())
@@ -145,7 +147,7 @@ class EventContext(object):
             event_data['res'] = {}
         event_dict = dict(event=event, event_data=event_data)
         for key in event_data.keys():
-            if key in ('job_id', 'ad_hoc_command_id', 'uuid', 'parent_uuid', 'created',):
+            if key in ('job_id', 'ad_hoc_command_id', 'project_update_id', 'uuid', 'parent_uuid', 'created',):
                 event_dict[key] = event_data.pop(key)
             elif key in ('verbosity', 'pid'):
                 event_dict[key] = event_data[key]