change stdout composition to generate from job events on the fly

this approach totally removes the process of reading and writing stdout
files on the local file system at settings.JOBOUTPUT_ROOT when jobs are
run; now stdout content is only written on-demand as it's fetched for
the deprecated `stdout` endpoint

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

awx/main/tests/unit/test_tasks.py

@@ -31,6 +31,7 @@ from awx.main.models import (
 )
 
 from awx.main import tasks
+from awx.main.queue import CallbackQueueDispatcher
 from awx.main.utils import encrypt_field, encrypt_value
 
 
@@ -199,6 +200,7 @@ class TestJobExecution:
         self.run_pexpect.return_value = ['successful', 0]
 
         self.patches = [
+            mock.patch.object(CallbackQueueDispatcher, 'dispatch', lambda obj: None),
             mock.patch.object(Project, 'get_project_path', lambda *a, **kw: self.project_path),
             # don't emit websocket statuses; they use the DB and complicate testing
             mock.patch.object(UnifiedJob, 'websocket_emit_status', mock.Mock()),