Command to list instances

`> opsicle instances <environment>`

```
+----------------------------------+-----------------------+--------+--------------------------------------+
| Hostname                         | Layers                | Status | Instance ID                          |
+----------------------------------+-----------------------+--------+--------------------------------------+
| member-management-staging-app-02 | Rails Puma App Server | online | fa85db6a-f863-490c-9352-15b03394fa96 |
| member-management-staging-util01 | Utility               | online | 9bc89715-8775-4f18-870b-93b75da55cc8 |
+----------------------------------+-----------------------+--------+--------------------------------------+
```