Prettify serf agent json (humans too read configuration files).

Because single-line configuration files get more and more difficult to read as they grow...

```json
{
  "event_handlers": [
    "/opt/serf/event_handlers/test-handler.sh"
  ],
  "bind": "0.0.0.0:7946",
  "start_join": [
    "192.168.0.11",
    "192.168.0.14",
    "192.168.0.10",
    "192.168.0.13",
    "192.168.0.12"
  ]
}
```