Changes for HOT hello world template processing

This patch contains the base enabling code required for processing a most
simple HOT template ("hot hello world"), so that the template can be
parsed and a stack with a single instance can be deployed.

Contributes to blueprint hot-hello-world

Change-Id: I4f0a04a06fab2f5472cb4481dcdf10d5239d5def
Merge "Changes for HOT hello world template processing"