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/centos-7.json b/packer/vars/centos-7.json
index aa629a9..340f545 100644
--- a/packer/vars/centos-7.json
+++ b/packer/vars/centos-7.json
@@ -1,6 +1,8 @@
 {
   "source_ami_filter_name": "*CentOS Linux 7*HVM*",
-  "source_ami_filter_owner": "679593333241",
+  "source_ami_filter_owner": "aws-marketplace",
+  "source_ami_filter_product_code": "aw0evgkw8e5c1q413zgy5pjce",
+
   "ssh_user": "centos",
 
   "distro": "CentOS 7",