Update image id

Updated the image ID. The one added is CoreOS Stable.

```
$ nova image-list | grep Core | grep Metal
| f08e095c-9098-4a10-bbed-0394b40bb90f | OnMetal - CoreOS (Alpha)
| 87b95448-111f-4406-a048-6f12f93ead5d | OnMetal - CoreOS (Beta)
| 85e3d8d2-6d0f-4ded-bd55-0f13d3e57e69 | OnMetal - CoreOS (Stable)
```