Quick return from create nodes if count 0
Quick return from create nodes if count 0

When count is 0, e.g. cluster.desired_capacity is set to 0, we don't
bother to execute any node creation logic. This patch provides a quick
path for return.

Change-Id: Ib31522f1a3024981bfbcd93f0df09f8ad0be963d

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