make test_really_large_instance_group pass (though I didn't really want it to...)

Change-Id: Ic7c83b468e2b3983a07605c473109b3484df89bf
refactor tons of resource_id usage into a helper.

I'll be able to replace this helper with one that inspects the
nested stack later.

Change-Id: I4f54ef7e19b68b89a04ac6cea5bae12054faf900