Add OpsWorks Custom Layer Hosts (/etc/hosts)

We need the ability to define hosts entries per layer. Here is a pull request as this already has been requested (https://github.com/aws/opsworks-cookbooks/issues/51).

```json
{
    "opsworks": {
        "layers": {
            "app": {
                "hosts": {
                    "127.0.0.1": "foobar.com.au"
                }
            }
        }
    }
}
```