Terraform Version Packaging

_I posted this question in gitter but am following up here..._

Is there a way to force Chalice to package for a certain version of terraform? The default behavior appears to package for Terraform 0.11.x as it formats variables like `${variable}` in the generated json file.

`chalice package --stage dev --pkg-format terraform /tmp/pkg/`

Additionally, as a feature request, it would be great to document best practices around Terraform upgrades as they relate to Chalice.

Start by reproducing `chalice package --stage dev --pkg-format terraform /tmp/pkg/` and inspect the generated JSON where Terraform variable syntax appears. Trace the Terraform packaging entry point to determine whether a target Terraform version can be selected, and review the existing documentation structure. Done means the supported version behavior is defined and Terraform upgrade guidance is documented.

Written by the indexing model from the issue text.