Submitted is _not_ necessarily merged in Gerrit

* zuul/trigger/gerrit.py(Gerrit._isMerged): Only consider MERGED
status to indicate a change is successfully merged. SUBMITTED can
exist on changes which Gerrit was unable to merge due to nonexistent
parent/dependent commits, and possibly for other reasons too.

Change-Id: I673be5010031f8bc26ab31db9afcc7c28854db85

zuul/trigger/gerrit.py

@@ -236,7 +236,7 @@ class Gerrit(object):
         if not status:
             return False
         self.log.debug("Change %s status: %s" % (change, status))
-        if status == 'MERGED' or status == 'SUBMITTED':
+        if status == 'MERGED':
             return True
 
     def canMerge(self, change, allow_needs):