Refactored cluster action implementation
Refactored cluster action implementation

Major changes include:
1. Use consts module for action name references;
2. Added '_create_nodes' function to be shared by action procedures;
3. Added '_delete_nodes' function to be shared by action procedures;
4. Implemented scale_out and scale_in methods;
5. Reworked other action procedures to make use of the utility methods;

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