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"
                }
            }
        }
    }
}
```
Result:
```bash
$ cat /etc/hosts
# This file was generated by OpsWorks
# any manual changes will be removed on the next update.

127.0.0.1 localhost localhost.localdomain

# The following lines are desirable for IPv6 capable hosts
# ...

# OpsWorks Layer State
127.0.0.1 test.localdomain test
11.11.0.111 app1
33.444.77.99 app1-ext

# Custom layer hostnames
127.0.0.1 foobar.com.au
```