Enforce unique descriptions for variable and output blocks

[evairmarinho]

In Terraform, is common copy and paste variables, only changing the type and the alias.

Example:

variable "db_username" {
type = string
description = "Username of database"
}

Crating a new variable for db password:

variable "db_username" {
type = string
description = "Username of database"
}

variable "db_password" {
type = string
description = "Username of database"
}

New rule for tflint

tflint --only=terraform_documented_variables_with_different_description

[Warning] Same description of terraform variables in variables.tf

[bendrucker]

This would fit fine as a feature of the existing variable rule. Some of the the rules could really use a rename. Like the rule name should be terraform_variable_description and the attribute here would be unique = true. That way the rule could also have required = true (by default). Users could choose to enforce unique descriptions but not require all variables have one.

I'll rename and deprecate a bunch of rules in one batch. Happy to review a PR adding the unique attribute to the existing rule with its existing name.