0.16.0 Make `ssl_support_method` empty when using CloudFront default certificate

what

• Make ssl_support_method empty when using CloudFront default certificate

why

• The current implementation always uses "sni-only" as ssl_support_method in viewer_certificate configuration.
  According to Terraform documentation this option is required only when using acm_certificate_arn or iam_certificate_id.
  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 and not if we are using CloudFront default certificate