Support deletion policy for stack-resource

We should support the deletion policy for stack-resources, such as autoscaling group, resource group, heat autoscaling group and so on:
1. the deletion policy of stack-resource is DELETE ----ok
2. the deletion policy of stack-resource is SNAPSHOT ----now will failed in resource.validate(), so to add handle_snapshot_delete(*) implementation for stack-resources, to snapshot for nested resources, and then to delete the nested stack and resources
3. the deletion policy of stack-resource is RETAIN ----now the records of nested stack and nested resources are residual in heat, may be we can implementation delete() for stack-resource