Fix: Add aws_partition data source.

[dertdog]

The ARNs should be created using a data source to determine the current partition.

data "aws_partition" "current" {}

With the data source, a resource can be created as follows:

    resources = [
      "arn:${data.aws_partition.current.partition}:s3:::${local.elb_logs_s3_bucket}/*",
    ]

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Addresses #66