add a new property "fast_scaling" to cluster

Add a new property "fast_scaling" to cluster, some standby nodes will create when create a cluster. With the new property set, senlin will scale out use standby nodes first and move nodes from cluster to standby nodes when scale in. The number of standby nodes is amount to max_size - desired_capacity.

The plan is:
1. Add a new attribute named "fast-scaling" to cluster, default is "False".
2. Revises cluster-create operation, if the attribute's value is "True", cluster will create standby nodes when creating.
3. Revises cluster-scale-out and cluster-scale-in operation, add standby nodes to cluster when perform scale out operation or move nodes from cluster to standby nodes when perform scale in operation.
4. Revises cluster-update operation, create or delete standby nodes if 'fast_scaling''s value changed.