Add a safety check on stack ARN when building

Support for specifying a `stack_name` separate from `name` was added in https://github.com/remind101/stacker/pull/551 to support stacks with the same name, but in different regions/accounts, however, this introduces the possibility for two nodes in the graph to operate on the same stack.

I think it'd be wise to add an extra layer of safety so that stacker will error out if two nodes in the graph operate on the same stack _ARN_, as this would almost always be programmer error (e.g. copy/paste but forget to change `stack_name`).