Add X-Github-Delivery id to debug logs

This is how one can look up a payload in the github web UI.

Change-Id: Ibaeb3b8b4e3482d34f2e3564f15bb6be4acdf02e
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index f31df6a..bac66f1 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -75,7 +75,9 @@
             raise webob.exc.HTTPMethodNotAllowed(
                 'Only POST method is allowed.')
 
-        self.log.debug("Github Webhook Received.")
+        delivery = request.headers.get('X-GitHub-Delivery')
+        self.log.debug("Github Webhook Received: {delivery}".format(
+            delivery=delivery))
 
         self._validate_signature(request)