Add support for terraform 0.12

[raymondberg]

Addresses: #40 Terraform 0.13 support

Problem

Terraform 0.12 totally rewrote their internal provider to use a different API. Upgrading steps are
not ideal, but documented here: https://www.terraform.io/docs/extend/terraform-0.12-compatibility.html

Given the direction of Terraform, upgrading this provider is critical to promote centralized management
and focused development. Until this is merged and new binaries release, using modern versions of
terraform (0.12-0.14) isn't possible despite some really awesome work done in this prior PR: #29

Solution

The meat of this PR represent the code changes needed to upgrade to 0.12. It's mostly as easy
as swapping out the underlying inherited interface and corresponding method signatures.

I refrained from proposing other changes like release steps for contributors, which I'd like to
improve on in a subsequent PR if acceptable.

Tests were passing on debian (assuming a merge of another PR: #43), and with it I was able to get
a terraform apply producing output from the module under terraform 0.12.21.