Enable questions to depend on prior answers

[josh-padnick]

Here's an excerpt from my vars.tf:

variable "kms_key_administrator_iam_arns" {
  description = "A list of the IAM ARNs (Users or Roles) that should have Administrator access to the KMS key created for this service. Administrators can manage the Key Policy and use it to encrypt/decrypt."
  default = "arn:aws:iam::1234567890:user/gus,arn:aws:iam::1234567890:user/jim,arn:aws:iam::1234567890:user/josh"
}

One issue with this is that this question / boilerplate variable should only appear if IsUsingKms is true.