create-amis: allow customizing the service role name

The complete setup on the AWS end can be configured
with the following Terraform configuration. It generates
a ./credentials.sh which I just copy/pasted in to the
create-amis.sh script near the top. Note: the entire stack
of users and bucket can be destroyed at the end of the
import.

    variable "region" {
      type = string
    }
    variable "availability_zone" {
      type = string
    }

    provider "aws" {
      region = var.region
    }

    resource "aws_s3_bucket" "nixos-amis" {
      bucket_prefix = "nixos-amis-"
      lifecycle_rule {
        enabled = true
        abort_incomplete_multipart_upload_days = 1
        expiration {
          days = 7
        }
      }
    }

    resource "local_file" "credential-file" {
      file_permission = "0700"
      filename = "${path.module}/credentials.sh"
      sensitive_content = <<SCRIPT
    export service_role_name="${aws_iam_role.vmimport.name}"
    export bucket="${aws_s3_bucket.nixos-amis.bucket}"
    export AWS_ACCESS_KEY_ID="${aws_iam_access_key.uploader.id}"
    export AWS_SECRET_ACCESS_KEY="${aws_iam_access_key.uploader.secret}"
    SCRIPT
    }

    # The following resources are for the *uploader*
    resource "aws_iam_user" "uploader" {
      name = "nixos-amis-uploader"
    }

    resource "aws_iam_access_key" "uploader" {
      user = aws_iam_user.uploader.name
    }

    resource "aws_iam_user_policy" "upload-to-nixos-amis" {
      user = aws_iam_user.uploader.name

      policy = data.aws_iam_policy_document.upload-policy-document.json
    }

    data "aws_iam_policy_document" "upload-policy-document" {
      statement {
        effect = "Allow"

        actions = [
          "s3:ListBucket",
          "s3:GetBucketLocation",
        ]

        resources = [
          aws_s3_bucket.nixos-amis.arn
        ]
      }

      statement {
        effect = "Allow"

        actions = [
          "s3:PutObject",
          "s3:GetObject",
          "s3:DeleteObject",
        ]

        resources = [
          "${aws_s3_bucket.nixos-amis.arn}/*"
        ]
      }

      statement {
        effect = "Allow"
        actions = [
          "ec2:ImportSnapshot",
          "ec2:DescribeImportSnapshotTasks",
          "ec2:DescribeImportSnapshotTasks",
          "ec2:RegisterImage",
          "ec2:DescribeImages"
        ]
        resources = [
          "*"
        ]
      }
    }

    # The following resources are for the *vmimport service user*
    # See: https://docs.aws.amazon.com/vm-import/latest/userguide/vmie_prereqs.html#vmimport-role
    resource "aws_iam_role" "vmimport" {
      assume_role_policy = data.aws_iam_policy_document.vmimport-trust.json
    }

    resource "aws_iam_role_policy" "vmimport-access" {
      role = aws_iam_role.vmimport.id
      policy = data.aws_iam_policy_document.vmimport-access.json
    }

    data "aws_iam_policy_document" "vmimport-access" {
      statement {
        effect = "Allow"
        actions = [
          "s3:GetBucketLocation",
          "s3:GetObject",
          "s3:ListBucket",
        ]
        resources = [
          aws_s3_bucket.nixos-amis.arn,
          "${aws_s3_bucket.nixos-amis.arn}/*"
        ]
      }
      statement {
        effect = "Allow"
        actions = [
          "ec2:ModifySnapshotAttribute",
          "ec2:CopySnapshot",
          "ec2:RegisterImage",
          "ec2:Describe*"
        ]
        resources = [
          "*"
        ]
      }
    }

    data "aws_iam_policy_document" "vmimport-trust" {
      statement {
        effect = "Allow"
        principals {
          type = "Service"
          identifiers = [ "vmie.amazonaws.com" ]
        }

        actions = [
          "sts:AssumeRole"
        ]

        condition {
          test = "StringEquals"
          variable = "sts:ExternalId"
          values = [ "vmimport" ]
        }
      }
    }

nixos/maintainers/scripts/ec2/create-amis.sh

@@ -6,9 +6,10 @@
 # <nixos/release.nix> amazonImage attribute. Images are uploaded and
 # registered via a home region, and then copied to other regions.
 
-# The home region requires an s3 bucket, and a "vmimport" IAM role
+# The home region requires an s3 bucket, and an IAM role named, by default,
-# with access to the S3 bucket.  Configuration of the vmimport role is
+# "vmimport" IAM role with access to the S3 bucket. The name can be
-# documented in
+# configured with the "service_role_name" variable. Configuration of the
+# vmimport role is documented in
 # https://docs.aws.amazon.com/vm-import/latest/userguide/vmimport-image-import.html
 
 # set -x
@@ -18,6 +19,7 @@ set -euo pipefail
 state_dir=$HOME/amis/ec2-images
 home_region=eu-west-1
 bucket=nixos-amis
+service_role_name=vmimport
 
 regions=(eu-west-1 eu-west-2 eu-west-3 eu-central-1 eu-north-1
          us-east-1 us-east-2 us-west-1 us-west-2
@@ -196,7 +198,7 @@ upload_image() {
 
         log "Importing image from S3 path s3://$bucket/$aws_path"
 
-        task_id=$(aws ec2 import-snapshot --disk-container "{
+        task_id=$(aws ec2 import-snapshot --role-name "$service_role_name" --disk-container "{
           \"Description\": \"nixos-image-${image_label}-${image_system}\",
           \"Format\": \"vhd\",
           \"UserBucket\": {