Terraform 0.12 Compatibility

[kmanning]

Update and support Terraform 0.12. For information on the breaking changes to plugins see:

https://www.terraform.io/docs/extend/terraform-0.12-compatibility.html

hashicorp/terraform#21454

TF 0.12: terraform validate -check-variables flag not defined

missing required provider arguments should be a WARNING not an ERROR

hashicorp/terraform#21408

[kmanning]

Work done so far by the PR above:

1. As of terraform 0.12 and above, terraform validate now needs a terraform init.
2. The PR above solves this problem by adding terraform init -backend=false to the TerraformValidateStage.
3. -backend=false is required for TerraformValidateStage, only because many/most of the backend plugins are environment-based, and TerraformValidateStage was never designed to have environment configuration.

Work left to do:

1. The before/after hooks in TerraformValidateStage are insufficient. You can only decorate around the entirety of delete/checkout/validate. Offer more flexibility here, to allow decorations around terraform validate itself.
2. Once that's available, remove the hardcoded terraform init introduced by the PR above.
3. Modify TerraformPlugin to conditionally inject terraform init for 0.12+. For versions prior to 0.12, leave the functionality as-is prior to the first PR for this issue (that is, prior to 0.12, TerraformValidate should NOT run terraform init -backend=false). This will remove the requirement that was initially added as part of this issue - we will NOT require You are using at least terraform 0.9.x (must support '-backend=false')

I'll make a separate PR to complete the left over work for this Issue.

[kmanning]

Released in v5.3 https://github.com/manheim/terraform-pipeline/releases/tag/v5.3