Add missing required field (terraform-aws-modules#81)

Field `owners` is required for `aws_ami`, otherwise it fails

Author: tiberiuana

examples/basic/main.tf

@@ -15,7 +15,8 @@ data "aws_subnet_ids" "all" {
 
 data "aws_ami" "amazon_linux" {
  most_recent = true
-
+ owners = ['amazon']
+ 
  filter {
  name = "name"