Introduce Resource Type that Produces a Webhook that Invokes Stack-Update-By-Deltas

Building on the blueprint that introduces a variant of stack update that takes deltas instead of a whole template, introduce a resource type that makes this operation available through a resource in a template. In particular, such a resource has properties that provide the arguments to the variant update operation, and has an attribute that is a webhook (signal URL) that, when invoked, invokes the variant update operation. All the variant update parameters are given in the resource's properties rather than the request body of the signal invocation because a Ceilometer alarm allows no user-specified content in the request body of an alarm action.