Terraform plugin SDK update

[poddm]

Looks like cloudstack is using the "legacy Terraform plugin SDK". To ensure terraform core compatibility we should look at upgrading to SDK v2.

Current: github.com/hashicorp/terraform v0.12.0

• https://github.com/apache/cloudstack-terraform-provider/blob/main/go.mod#L9

v1-upgrade-guide

As of September 2019, Terraform provider developers importing the Go module github.com/hashicorp/terraform, known as Terraform Core, should switch to github.com/hashicorp/terraform-plugin-sdk, the Terraform Plugin SDK, instead.
..
We will use the term "legacy Terraform plugin SDK" when referring to the version of Terraform Core imported and used by providers.

Upgrading to the latest Terraform SDK

At the time of the Terraform 0.12 release, the Terraform SDK is a set of sub-directories inside the Terraform Core repository. Therefore upgrading to the latest Terraform SDK involves upgrading all of the dependencies on Go packages with the prefix github.com/hashicorp/terraform/ to a version with support for the new provider protocol.

NOTE: The SDK is now its own Go module. Existing providers should upgrade to at least v0.12.7 before switching to the standalone SDK. This is to isolate issues between Terraform SDK v0.11 and v0.12, and the standalone SDK. New providers should vendor the standalone SDK from the start.

[fabiomatavelli]

This was implemented by #71