Write CloudFront Access Logs to Single S3 Bucket

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Having CloudFront write its access logs to a bucket has been introduced in #5.

At the moment, a new S3 bucket gets created for every serverless static website instance created by the Terraform module. Ironically, the CloudFront access logs for subdomains are already prefixed within the S3 bucket and hence put in their corresponding "directory". This forced creation of a new S3 logging bucket doesn't really seem to be necessary.

It would be nice to specify the S3 bucket used for CloudFront access logging and hence have all logs in one central S3 bucket. By definition of subdomains and domains, there shouldn't be any clashes.

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Closing this for now until a better solution for creating optional resources is available

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As long as hashicorp/hil#50 has not been fixed, there are essentially two options to solve this:

1. Accept the fact that a logging bucket always gets created (even though it's not used and not requested in the input variables)
2. Make the log_bucket_domain_name a mandatory input but provide an additional Terraform module to easily create a logging bucket

As of now, it seems like 2 is the best option.

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Reopening in order to implement solution 2 described above.