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