tags.sh

#!/bin/bash

eval "$(jq -r '@sh "ID=\(.id) TAG_KEY=\(.tag_key) TAG_VALUE=\(.tag_value)"')"

aws ec2 create-tags --resources $ID --tags "Key=$TAG_KEY,Value=$TAG_VALUE"

jq -n --arg subnet_id "$ID" \
 --arg tag_key "$TAG_KEY" \
 --arg tag_value "$TAG_VALUE" \
 '{"subnet_id": $subnet_id, "tag_key": $tag_key, "tag_value": $tag_value}'

vpc_tags.tf

data "external" "add_tag_to_public_subnets" {
  count = "${length(module.aws_networks.public_subnet_ids)}"
  program = [ "bash", "${path.cwd}/tags.sh" ]
  query = { 
   id = "${element(module.aws_networks.public_subnet_ids, count.index)}"
   tag_key = "${format("kubernetes.io/cluster/%s", var.CLUSTER_NAME)}"
   tag_value = "shared"
  }
}

output "test" { 
  value = "${data.external.add_tag_to_public_subnets.*.result}"  
}