Log item warning messages at info level

In some cases, Zuul records warning messages to accompany the
build report.  These typically constitute decisions Zuul has made
which affect execution (or non-execution) of jobs, so log them
at info level so that admins have at least as much context as
end-users.

Change-Id: I73a0f6d7878873c259207a75d8b8ba1374ddc7b7

zuul/model.py

@@ -2191,6 +2191,7 @@ class QueueItem(object):
 
     def warning(self, msg):
         self.current_build_set.warning_messages.append(msg)
+        self.log.info(msg)
 
     def freezeJobGraph(self, skip_file_matcher=False):
         """Find or create actual matching jobs for this item's change and