Fix Lambda path in shared state (#17)

* Cirumvent TF's path.module limitation for lambda filenames

main.tf

@@ -28,10 +28,22 @@ resource "aws_lambda_permission" "sns_notify_slack" {
   source_arn    = "${local.sns_topic_arn}"
 }
 
+data "null_data_source" "lambda_file" {
+  inputs {
+    filename = "${substr("${path.module}/functions/notify_slack.py", length(path.cwd) + 1, -1)}"
+  }
+}
+
+data "null_data_source" "lambda_archive" {
+  inputs {
+    filename = "${substr("${path.module}/functions/notify_slack.zip", length(path.cwd) + 1, -1)}"
+  }
+}
+
 data "archive_file" "notify_slack" {
   type        = "zip"
-  source_file = "${path.module}/functions/notify_slack.py"
-  output_path = "${path.module}/functions/notify_slack.zip"
+  source_file = "${data.null_data_source.lambda_file.outputs.filename}"
+  output_path = "${data.null_data_source.lambda_archive.outputs.filename}"
 }
 
 resource "aws_lambda_function" "notify_slack" {