Added conditional (#9)

* Output variable was referencing incorrect value

* Removed option that sets cloudfront_default_certificate
- According to terraform documentation you either have to set cloudfront_default_certificate or acm_certificate_arn for viewer_certificate in the aws_cloudfront_distribution resource, but not both.

* Added a conditional to viewer_certificate block
- This was needed because if you set acm_certificate_arn then cloudfront_default_certificate needs to be false otherwise it needs to be true.

main.tf

@@ -128,7 +128,7 @@ resource "aws_cloudfront_distribution" "default" {
     acm_certificate_arn            = "${var.acm_certificate_arn}"
     ssl_support_method             = "sni-only"
     minimum_protocol_version       = "TLSv1"
-    cloudfront_default_certificate = true
+    cloudfront_default_certificate = "${var.acm_certificate_arn == "" ? 1 : 0}"
   }
 
   default_cache_behavior {