This unifies the 'lightweight stack' used in convergence with the how
things work the rest of the time: we now always obtain resource data from
the StackDefinition's ResourceProxy objects. This means that when we are
checking an individual resource, we will never create all of the other
Resource objects for the stack in memory (although we already avoided
loading data for them from the database) - now we will only create
ResourceProxy objects as needed.
Change-Id: Id7472557e26d172df88841ff7f20afdd7f5bfada
Implements: blueprint stack-definition
764b8fb251