Bad structure for propella-4b section in metadata

Propella section can look like this:
```yaml
propella-4b: 
    hash: sha256
    hash-id: id
    high/actual:
```

It is confusing since it is not clear what are configurations and what are parts.

A better structure more alligned with the rest is:
```yaml
propella-4b: 
    default:
        hash: sha256
        hash-id: id
    high/actual:
```