fix find placeholder stack so config/terraform only builds for stacks

[tongueroo]

This is a 🐞 bug fix.

• 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

Fixes terraspace build so that it won't accidentally build config/terraform files to modules folder when the placeholder stack that is found happens to a be a module. Placeholder stacks should only be stacks.

Context

Fixes #66

How to Test

Go through QA matrix.

Version Changes

Patch