add var.prefix to aws_iam_role name

[hall]

Since this module is meant to be created per EKS cluster, this change allows having multiple clusters in the same AWS account, each with their own IAM role.

[hall]

Oh, I do want to point out that this is a breaking change and will require users to update the role name in the Expel dashboard.

[hall]

Since this is a breaking change, it might also be worth changing the IAM role casing as it doesn't match that of the kinesis data stream and cloudwatch subscription filter.

However, it would be nice to shorten some of these hard-coded suffices (or altogether remove the ones which are redundant like -kinesis-data-stream on a Kinesis data stream) to increase the max limit on the length of var.prefix (though I'm not entirely sure which resource name that limit is coming from).