Major rework of the deletion policy
Major rework of the deletion policy

Changes include:
1. Add inline document showing how policy works;
2. Add support to 'OLDEST_PROFILE_FIRST' as a criteria;
3. Add support 'destroy_after_delete' as an option;
4. Reworked candidate selection into a utility method;
5. Reworked pre_op() and praying it will work.

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