Transform support for nested stacks

[benkehoe]

Nested stacks should be able to use transforms, like AWS SAM, in exactly the way top-level stacks do. This is blocking AWS SAM CLI issue aws/aws-sam-cli#1213. Transform support for StackSets is open as #207.

[jmacgowa-ks]

We are also affected by this. We use a lot of "{{ssm:resolve:}}" SSM parameter references within our templates. Instead of manually updating the versions referenced within the templates, we use a macro to find the latest version of each of them, as the "ssm:resolve" feature doesn't support a latest version option.

As we use nested stacks sparingly, we are able to work around this for now.

[uberllama]

Us as well.