Update elasticsearch_install.rb

Modified to allow IAM Roles.
As per: https://github.com/elasticsearch/elasticsearch-cloud-aws
The plugin will default to using IAM Role credentials for authentication. These can be overridden by, in increasing order of precedence, system properties aws.accessKeyId and aws.secretKey, environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_KEY, or the elasticsearch config using cloud.aws.access_key and cloud.aws.secret_key

If no IAM role is used, and access and secret key are not set, the autodiscovery process will fail.