Terragrunt: Add "prevent destroy" to Terragrunt

[brikis98]

Terragrunt should support a prevent_destroy flag that, if set to true, will prevent terraform destroy from being executed. This is useful for modules you want to carefully protect, such as a database, or a module that provides auth (which isn't a "dependency" of other modules, but if you delete it, you won't be able to access anything else!).

terragrunt = {
  prevent_destroy = true
}

[brikis98]

Added in #525.