Option to for kustomize substitution to fail for undefined variables

[jleni]

At the moment, kustomize substitutions works great but according to documentation:

All the undefined variables in the format ${var} will be substituted with an empty string unless a default value is provided e.g. ${var:=default}.

This is important to avoid unexpected results when default values as not possible.

We are happy to contribute with a PR and make changes here (and related code) https://github.com/fluxcd/pkg/blob/6637febc3169ea28bf2073e9239d8217013c0c72/kustomize/kustomize_varsub.go#L55