Handle debug messages cleanly

This changes from:

  2017-09-06 18:17:37.173853 | TASK [debug]
  2017-09-06 18:17:37.216661 | localhost | ok:
  2017-09-06 18:17:37.216882 | localhost | {
  2017-09-06 18:17:37.216967 | localhost |   "msg": "Job triggered from https://review.openstack.org/#/c/501485"
  2017-09-06 18:17:37.217047 | localhost | }

to:

  2017-09-06 18:20:34.383995 | TASK [debug]
  2017-09-06 18:20:34.429648 | Job triggered from https://review.openstack.org/#/c/501485

Change-Id: Iac39124c2a3b286215d175f98742d00ec5a12778

playbooks/zuul-stream/fixtures/test-stream.yaml

@@ -10,6 +10,10 @@
       debug:
         var: setupvar
 
+    - name: Output a debug sentence
+      debug:
+        msg: This is a debug message
+
     - name: Run a shell task
       command: ip addr show
 

zuul/ansible/callback/zuul_stream.py

@@ -374,9 +374,20 @@ class CallbackModule(default.CallbackModule):
             for key in [k for k in result_dict.keys()
                         if k.startswith('_ansible')]:
                 del result_dict[key]
-            self._log_message(
-                msg=json.dumps(result_dict, indent=2, sort_keys=True),
-                status=status, result=result)
+            keyname = next(iter(result_dict.keys()))
+            # If it has msg, that means it was like:
+            #
+            #  debug:
+            #    msg: Some debug text the user was looking for
+            #
+            # So we log it with self._log to get just the raw string the
+            # user provided.
+            if keyname == 'msg':
+                self._log(msg=result_dict['msg'])
+            else:
+                self._log_message(
+                    msg=json.dumps(result_dict, indent=2, sort_keys=True),
+                    status=status, result=result)
         elif result._task.action not in ('command', 'shell'):
             if 'msg' in result_dict:
                 self._log_message(msg=result_dict['msg'],