Feature request: add new cli option, like "-var-file" but with file interpolation

[vazhnov]

Hi, Terraform team!

I think it should be useful to have option like "-var-file" but with interpolation of file with variables.
My use case — store various AWS credentials for multiple application, I have to substitute credentials inside apps options. It will be good to store secrets in one file and substitute them in another files.

Var file example:
env_vars = ["JAVA_OPTS", "-Xmx6G -DAWS_ACCESS_KEY_ID=${var.api.aws_key} -DAWS_SECRET_KEY=${var.api.aws_secret}"]

Workaround: https://github.com/jonatanblue/terraform-environment-secrets

[vazhnov]

Related problem: #4084