clush: simplify the noderange printed

Hello,

Clusters often end in messy states that requires to run commands on a noderange full of holes like:

```
fortoy[200-203,205-233,235-345,347-500,...]
```

It could be nice to introduce an option that optimize the noderange printed as result, using groups and exclusion. For instance with the above example:

```
@compute -x fortoy[204,234,346]
```

Regards,
Xavier
