fix(output): Rename "role_arn" output to "role_id"

[k911]

Output role_arn was wrongly named, because it exposes "aws_iam_role.role.id" attribute
instead of "aws_iam_role.role.arn".

I've added both role_arn and role_id, because in my case I've needed
role_arn.

Note: This change is backwards incompatible, and will require changes
when making upgrade of modules using this module.

Example:
https://github.com/cloudposse/terraform-aws-ecs-codepipeline/blob/master/main.tf#L198

resource "aws_iam_role_policy_attachment" "codebuild_s3" {
  count      = "${local.enabled ? 1 : 0}"
-  role       = "${module.build.role_arn}"
+  role       = "${module.build.role_id}"
  policy_arn = "${aws_iam_policy.s3.arn}"
}

[aknysh]

thanks @k911

Looks good, but please rebuild README by executing these commands:

make init
make readme/deps
make readme

It will add the new variables and outputs to README.md automatically.

In general, any changes to README should be made in README.yaml (not in this case), and after that executing the commands above will rebuild README.yaml into README.md and add all new variables and outputs to README.md

thanks

[k911]

@aknysh Nice, I've must have missed this, thanks. I've updated PR according to your instructions.

[aknysh]

thanks @k911