[WIP] Functionality for adding external domain to the site

[stekern]

Adds support for associating multiple domain names with the CloudFront distribution and ACM certificate. The changes introduced are not backwards-compatible.

Due to issues with the AWS provider, issues arise when you add/remove domains after the code has already been applied (the domains in aws_acm_certificate.cert_website.domain_validation_options will not be up-to-date when applying, so an Index/Element error will be raised). A temporary fix is to use Terraform's lookup method to fall back to known values if the domain_validation_options are not up-to-date. The fallback values should actually result in errors if used, but in practice the domain_validation_options are always up-to-date during the apply.

There are many issues with the aws_acm_certificate resource, and it is due for a rewrite in v3.0 (see issue hashicorp/terraform-provider-aws#13053).

Relevant issues:

• aws_acm_certificate "completes" too soon with incorrect state hashicorp/terraform-provider-aws#9840
• aws_route53_record.test_cert_validation issue with aws provider hashicorp/terraform-provider-aws#9596
• Provide a resource which is an ACM certificate with builtin route53 DNS validation hashicorp/terraform-provider-aws#9589

[colincoleman]

Due to the breaking changes and the temporary workarounds necessary to get this working. I suggest a new module (aws_terraform_multi_domain_static_site)? is created and referenced it in the README of this one.

[stekern]

A separate module has been set up here instead https://github.com/nsbno/terraform-aws-multi-domain-static-site/