Use a relative path to prevent spurious terraform changes.

The terraform filename parameter encodes an absolute path to the module,
which can be different when run on different machines. This changes it
to key on a relative path, per the recommendation here:

hashicorp/terraform#7613 (comment)

terraform/lambda.tf

@@ -8,9 +8,15 @@ locals {
   sg_ids = ["${element(concat(aws_security_group.lambda.*.id, list("")), 0)}"]
 }
 
+data "null_data_source" "lambda_file" {
+  inputs {
+    filename = "${substr("${path.module}/es-cleanup.zip", length(path.cwd) + 1, -1)}"
+
+  }
+}
 
 resource "aws_lambda_function" "es_cleanup" {
-  filename         = "${path.module}/es-cleanup.zip"
+  filename         = "${data.null_data_source.lambda_file.outputs.filename}"
   function_name    = "${var.prefix}es-cleanup${var.suffix}"
   description      = "${var.prefix}es-cleanup${var.suffix}"
   timeout          = 300