Fix UnboundLocalError when updating a cluster without nodes
Fix UnboundLocalError when updating a cluster without nodes

Change-Id: Idcd814bf90e2de20495929619497fc6c4377c98c
Closes-Bug: #1481265

Alzon/senlin,tengqm/senlin-container,stackforge/senlin,openstack/senlin,Alzon/senlin,tengqm/senlin-container,openstack/senlin,openstack/senlin,stackforge/senlin