Extract Terraform CLI interactions to transport plugin

[aaron-lane]

Functions

It will:

• handle Terraform CLI interactions

Objectives

It should:

• optionally verify support for the Terraform CLI version
• ensure the path to the Terraform CLI is valid (fixes Path to Terraform CLI is not validated #120)

Constraints

It must:

• implement the existing workflow provided by the driver

[robbwagoner]

This should also support the cli attribute/key to allow use of Terraform wrappers like Terragrunt (since it is no longer supported in the Terraform driver).

[aaron-lane]

Fixed by #500. @robbwagoner that behaviour is still supported by the client attribute.