Empty ssl_support_method when using cloudfront default certificate (#46)

The current implementation always uses "sni-only" as
`ssl_support_method` in `viewer_certificate` configuration.
According to Terraform documentation [0] this option is required
only when using `acm_certificate_arn` or `iam_certificate_id`.
In our experience this leads to a situation where Terraform
tries to set `ssl_support_method` to "sni-only" at each
run spending several time trying to do it (~10 minutes) without
effectively setting anything (it doesn't fail though).
With this commit we check the value of `acm_certificate_arn` and set the
proper `ssl_support_method` only if such value is defined.

[0] https://www.terraform.io/docs/providers/aws/r/cloudfront_distribution.html#ssl_support_method

Co-authored-by: Andriy Knysh <[email protected]>

main.tf

@@ -189,7 +189,7 @@ resource "aws_cloudfront_distribution" "default" {
 
   viewer_certificate {
     acm_certificate_arn            = var.acm_certificate_arn
-    ssl_support_method             = "sni-only"
+    ssl_support_method             = var.acm_certificate_arn == "" ? "" : "sni-only"
     minimum_protocol_version       = var.minimum_protocol_version
     cloudfront_default_certificate = var.acm_certificate_arn == "" ? true : false
   }