Use Product Code to filter AMIs used as source images by packer

Add documentation for this process

Change-Id: I5d548ceefb93602d751b5c88230e7dba0a88be2a
diff --git a/packer/vars/ubuntu-16.04.json b/packer/vars/ubuntu-16.04.json
index 972ff75..4f7d473 100644
--- a/packer/vars/ubuntu-16.04.json
+++ b/packer/vars/ubuntu-16.04.json
@@ -1,6 +1,7 @@
 {
   "source_ami_filter_name": "*ubuntu*16.04*",
-  "source_ami_filter_owner": "099720109477",
+  "source_ami_filter_owner": "aws-marketplace",
+  "source_ami_filter_product_code": "csv6h7oyg29b7epjzg7qdr7no",
 
   "ssh_user": "ubuntu",