data "aws_vpc" "common" {
  filter {
    name   = "tag:Name"
    values = ["common"]
  }
}

data "aws_subnet" "common" {
  count = "${var.availability_zone_count}"
  vpc_id = "${data.aws_vpc.common.id}"
  availability_zone = "${element(module.region_variables.availability_zones, count.index)}"
}

resource "aws_autoscaling_group" "main" {
  ...
  availability_zones   = ["${data.aws_subnet.common.*.availability_zone}"]
  vpc_zone_identifier  = ["${data.aws_subnet.common.*.id}"]
  ...
}