change default fallback mod strategy to Mod::Tf instead of Mod::Pass for ERB support

[tongueroo]

This is a 🐞 bug fix.
This is a 🙋‍♂️ feature or enhancement.

• I've added tests (if it's a bug, feature or enhancement)
• I've adjusted the documentation (if it's a feature or enhancement)
• The test suite passes (run bundle exec rspec to verify this)

Summary

Caught this when using a terraform.tfvars file in a stack. By default, files should be using the ERB strategy.

How to Test

Create a terraform.tfvars with some ERB in a stack. And then run:

terraspace build demo

Context

Related: #158

Version Changes

Patch