Stack Made from Literal Template

Introduces resource type OS::Heat::ImediateStack, which creates a nested stack created from a template literal given in a property. This is useful where some other resource (e.g., new style scaling group) requires a single resource but the user wants an assembly of resources. This is more urgently needed when the containing template is generated by code that has no easy way to stash the desired assembly behind a file or URL. An example is the implementation of https://blueprints.launchpad.net/heat/+spec/asg-with-health .