Terraform 0.12 🚀

[jlsan92]

This is the PR for the migration to Terraform 0.12. 🎉

So far will be marked as WIP since the 0.12 still in beta and most likely the CI will fail 🚨

Before merge ⚠️

• Merge Terratest: Basic usage test #23 and rebase this PR
• Switch back to official Docker image
• Enable TFLint again (eventually) => WORK IN PROGRESS: A simple, unfinished implementation of a plugin system. terraform-linters/tflint#255

Closes: #35

[jlsan92]

Note: terraform validate disabled in CI because flag -check-variables flag seems to be removed from the command. I don't know if it's a bug or is intentionally removed, but in current docs the flag still there => https://www.terraform.io/docs/commands/validate.html#check-variables-true

Ref: hashicorp/terraform#21454

UPDATE: It seems that it's a bug in the cmd itself. Because terraform validate >= TF 0.12 has the -check-variables flag removed.

hashicorp/terraform#21408 (comment)