add support for draft-published event

Gerrit 2.4 did not provide a stream event for draft patchsets therefore it was not
supported in zuul either.  We've just upgraded to Gerrit 2.8 which does provide a
new draft-published stream event.  This change will make zuul aware of the draft-published
event.

Change-Id: Ica363b4672cfce375d64463636d84e92c2949fd6
Partial-Bug: #1255166
diff --git a/zuul/layoutvalidator.py b/zuul/layoutvalidator.py
index 1984aaa..7256bc4 100644
--- a/zuul/layoutvalidator.py
+++ b/zuul/layoutvalidator.py
@@ -45,6 +45,7 @@
 
     gerrit_trigger = {v.Required('event'):
                       toList(v.Any('patchset-created',
+                                   'draft-published',
                                    'change-abandoned',
                                    'change-restored',
                                    'change-merged',