Refactor message generator

* Job object can now control the output and generate K:V output for
  notification types that can support it
* Notifications store the body as json/dict now to encode more
  information
* Notification Type can further compose the message based on what is
  sensible for the notification type
* This will also allow customizing the message template in the future
* All notification types use sane defaults for the level of detail now

awx/main/notifications/email_backend.py

@@ -18,3 +18,10 @@ class CustomEmailBackend(EmailBackend):
     recipient_parameter = "recipients"
     sender_parameter = "sender"
 
+    def format_body(self, body):
+        body_actual = "{} #{} had status {} on Ansible Tower, view details at {}\n\n".format(body['friendly_name'],
+                                                                                         body['id'],
+                                                                                         body['status'],
+                                                                                         body['url'])
+        body_actual += pprint.pformat(body, indent=4)
+        return body_actual