Fixed node join/leave transaction
Fixed node join/leave transaction

These two operations will have to modify the custer's size and the node
index accordingly.  We wrap these operations into DB transactions to
ensure data consistency.

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