Allow 'MAY' in Gerrit submit records

If a review label uses NoOp, then it will indicate 'MAY' as the
status in the submit record for that label (indicating that there
are no criteria for that label to be satisfied).  Treat that in
the same way as OK so that a merge can occur.

Change-Id: I24be23d16aa133a3323b79bd775d42678ebc1dfe
diff --git a/zuul/trigger/gerrit.py b/zuul/trigger/gerrit.py
index c28401c..175e3f8 100644
--- a/zuul/trigger/gerrit.py
+++ b/zuul/trigger/gerrit.py
@@ -272,7 +272,7 @@
                     return True
                 elif sr['status'] == 'NOT_READY':
                     for label in sr['labels']:
-                        if label['status'] == 'OK':
+                        if label['status'] in ['OK', 'MAY']:
                             continue
                         elif label['status'] in ['NEED', 'REJECT']:
                             # It may be our own rejection, so we ignore