Fix subscriber input variable handler

[ImperialOps]

what

Adds a valid key of type string for sns subscriber handler

why

A recent commit adds defaults to subscriber input but also breaks any input on map key type:

Proposed fix tested with following input, pulled from cloudposse/terraform-aws-sns-topic:

locals {
  subscribers = {
    opsgenie = {
      protocol = "https"
      endpoint = "https://api.example.com/v1/"
      endpoint_auto_confirms = true
    }
  }
}

module "test" {
  source = "../terraform-aws-config"
  # Cloud Posse recommends pinning every module to a specific version
  # version     = "x.x.x"

  create_sns_topic = true
  create_iam_role  = true

  subscribers = local.subscribers
}

thanks

Thank you for these modules! ❤

[snowsky]

James and I found this PR is valid. Could you please re-open the PR?