Terraform: How to use a variable inside of another variable?
Largely inspired by the documentation the idea is to create a terraform datasource from a filter of the available resources and then use this as entry point in your ami resource:

variable "security_group_id" {}
variable "subnet_id" {}

data "aws_security_group" "selected" {
  filter {
    name = "${var.ansible_ami["security_groups_name"]}}"
  }
}

data "aws_subnet" "selected" {
  filter {
    name = "${var.ansible_ami["subnet_name"]}"
  }
}

resource "aws_instance" "ansible" {
    ami = "${var.ansible_ami["ami"]}"
    instance_type = "${var.ansible_ami["size"]}"
    security_groups = "${data.aws_security_group.selected.id}"
    subnet_id = "${data.aws_subnet.selected.id}"
    associate_public_ip_address = "${var.ansible_ami["enable_public_ip"]}"
    #user_data = ""


I'm not using terraform myself, so this could be plain wrong.