Handle upgrade of autohold held nodes

An earlier change alters the autohold held node IDs from a list
to a dict structure. If someone upgrades to this change, but has
existing autoholds with the old format, we will break them. This
change handles both cases.

Change-Id: Ib6381e459a9694cae61ba3229d48ddfe8f55f392

zuul/zk.py

@@ -592,7 +592,15 @@ class ZooKeeper(object):
         def getHeldNodeIDs(request):
             node_ids = []
             for data in request.nodes:
-                node_ids += data['nodes']
+                # TODO(Shrews): Remove type check at some point.
+                # When autoholds were initially changed to be stored in ZK,
+                # the node IDs were originally stored as a list of strings.
+                # A later change embedded them within a dict. Handle both
+                # cases here to deal with the upgrade.
+                if isinstance(data, dict):
+                    node_ids += data['nodes']
+                else:
+                    node_ids.append(data)
             return node_ids
 
         failure = False