Good Health via Stack Update-By-Deltas and Scaling Group Excluded Member Property

This blueprint describes how OS::Heat::AutoScalingGroup can maintain member health, as determined for a member's OS::Neutron::PoolMember, via the stack-updater-by-deltas resource type and the excluded_members property of the group. Each group member will be a resource with some ID (you might think of this as a name, but see http://docs.openstack.org/developer/heat/template_guide/hot_spec.html#resources-section), call it X, and a defining snippet that will be a nested stack that contains: (a) an instance of the user-provided resource snippet, which necessarily includes an OS::Neutron::PoolMember and exposes its ID; (b) a stack-updater-by-deltas that updates the scaling group resource (in its parent stack) by adding X to the list of excluded member IDs; (c) a Ceilometer alarm that reacts to sufficiently persistent bad health readings from (a)'s PoolMember by invoking the webhook of (b). The upward references needed by (b) are discoverable by the resource group code, which puts them in the appropriate places in the template it generates.