Test coverage for NestedStack, and fix FnGetRefId

NestedStack now has 100% coverage. resource_id is now set during create
instead of evaluated during FnGetRefId

Fixes: Bug #1131534
Change-Id: I21966acc711a801d28a8a22c377758d2363dd096
Merge "Test coverage for NestedStack, and fix FnGetRefId"