Fix BrooklyCatalog.createSpec, was creating the spec as if a regular app, not catalog item.

BrooklynAssemblyTemplateInstantiator.createSpec couldn't realize that the passed in plan was created from a catalog item - it was checking with the wrong id (plan name instead of catalog id). Fix the problem by passing a loader explicitly, making it the caller's responsibility to decide if it is a catalog item plan or a regular plan.