Investigate support for deleting stacks that have been removed from configuration

Adding a stack to a config file, deploying it, then removing it will currently leave it "orphaned" if you forget to run `stacker destroy` beforehand. It can, of course, be manually deleted using the AWS Console, but it's easy to miss it when you have hundreds of stacks.

It would be nice to have some way to enforce that a config contains all the stacks that represent one service. It would pair nicely with adding Nested Stacks support, such that a config could be represented by the parent stack, with all contained stacks as its children. Updates would go through the parent stack and remove any stacks that are no longer present.