Migrate node information to nodeset instead of nodes

Nodeset is the term now. For all the cases.

Change-Id: Ib58bdfa1744b2851ae6c6b79ece52600d905f53e
diff --git a/zuul/cmd/migrate.py b/zuul/cmd/migrate.py
index 05278aa..1d0870d 100644
--- a/zuul/cmd/migrate.py
+++ b/zuul/cmd/migrate.py
@@ -841,7 +841,10 @@
             output['vars']['BUILD_TIMEOUT'] = str(timeout * 1000)
 
         if self.nodes:
-            output['nodes'] = self.getNodes()
+            if len(self.nodes) == 1:
+                output['nodeset'] = self.getNodes()[0]
+            else:
+                output['nodeset'] = dict(nodes=self.getNodes())
 
         if expanded_projects:
             output['required-projects'] = list(set(expanded_projects))