Sort autoholds by request ID

Return the hold request IDs returned from the zookeeper API  as
a sorted list so that they will appear in sorted order in the
zuul CLI output.

Change-Id: I3a3d738ac2bebb8b446cb0710bf9f5452c232372

zuul/zk.py

@@ -544,7 +544,7 @@ class ZooKeeper(object):
         Get the current list of all hold requests.
         '''
         try:
-            return self.client.get_children(self.HOLD_REQUEST_ROOT)
+            return sorted(self.client.get_children(self.HOLD_REQUEST_ROOT))
         except kze.NoNodeError:
             return []