Feature request: do not raise an error when there are no changes

Deploying a stack with no changes currently produces an error:
error creating changeset: The submitted information didn't contain changes. Submit different information to create a change set.

In the context of CI/CD it would be great to have a flag (e.g. --allow-no-changes) that would make Rain ignore this error and consider the operation successful.

[stilvoid]

Hi @romain-depres I actually think it would be better if Rain were to exit with a success code in this case always. I'll look into that.