Migrate Terratest to go modules

[MatthiasScholz]

Summary

Terratest recently added support for the new dependency management system in go.

Hence we can migrate from go dep to go modules.

References

• https://github.com/gruntwork-io/terratest/releases/tag/v0.21.0
• https://blog.golang.org/migrating-to-go-modules

[MatthiasScholzTW]

Changes merged.