Add plan-patch for AWS DNS delegation

plan-patches/dns-delegation-aws/README.md

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+## AWS DNS Delegation
+
+If you have a DNS parent zone, this plan-patch creates the required "NS" record for DNS delegation. Can be used if `parent_zone` is empty, see:
+https://github.com/cloudfoundry/bosh-bootloader/blob/2a4d71fd093a77f6895e411c56fa6c14329b9d3f/terraform/aws/templates/cf_dns.tf#L5
+```
+cp -r bosh-bootloader/plan-patches/dns-delegation-aws/. some-env/
+bbl up
+```

plan-patches/dns-delegation-aws/terraform/aws_parent_ns.tf

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+variable "parent_zone_id" {
+  type        = "string"
+  description = "The AWS Route53 hosted zone ID for the 'parent' of the zone that bbl will create, used to set up DNS delegation"
+}
+
+resource "aws_route53_record" "perf-test" {
+  name            = "${var.system_domain}"
+  ttl             = 172800
+  type            = "NS"
+  zone_id         = "${var.parent_zone_id}"
+
+  records = ["${aws_route53_zone.env_dns_zone.name_servers}"]
+}