template.json

{
  "variables": {
    "salt_role": "common"
  },
  "provisioners": [
    {
      "type": "shell",
      "scripts": [
        "scripts/ec2-ephemeral-tmp.sh"
      ],
      "override": {
        "amazon-ebs": {
          "execute_command": "SALT_ROLE={{variable 'salt_role' }} sudo -E sh '{{.Path}}'"
        }
      }
    }
  ],
  "builders": [
    {
      "type": "amazon-ebs",
      "access_key": "XYZ",
      "secret_key": "XYZ",
      "region": "eu-west-1",
      "source_ami": "ami-6975691d",
      "instance_type": "m3.xlarge",
      "ssh_username": "ubuntu",
      "security_group_id": "default",
      "ami_name": "Base {{timestamp}}"
    }
  ]
}