[#66667110] Adds command to list instances attached to a load balancer

For the maintenance app we need a command to list all the machines attached to a certain load balancer.

```
$ elba help attached
Usage:
  elba attached BALANCER

Options:
  p, [--porcelain]  # Prints the list in a machine readable format

Prints the list of instances attached to a balancer
```

```
$ elba attached testing-loadbalancer
 * testing-loadbalancer
   - i-886613c1
   - i-fe6613b7
```