var.source_bucket_name should be respected as the S3 bucket name and not the prefix

[Chili-Man]

When setting var.source_bucket_name, it turns out that it's only setting the bucket name prefix (see here) and not the actual name of the bucket. I'm not sure why the behavior is different given the variable's name and description. The other name variables, var.name and var.iam_role_name, are respected and passed through as the names for those resources, so I'm not sure why it's inconsistent for the s3 bucket. Is this something that y'all would be open to? I am more than happy to contribute this change.

I understand that this proposal will be a breaking change, so we could have another variable var.source_bucket_name_prefix that preserves the original behavior.

[Chili-Man]

I've opened a pull request to tackle this here: #47