Add new feature to read version from terraform file

[warrensbox]

Use version.tf file

If a .tf file with the terraform constrain is included in the current directory, it should automatically download or switch to that terraform version. For example, the following should automatically switch terraform to version 0.12.24:

terraform {
  required_version = ">= 0.12.9"

  required_providers {
    aws        = ">= 2.52.0"
    kubernetes = ">= 1.11.1"
  }
}