Override manifest parameters via CLI options

It will be useful to have options to override parameters of `genesis.yaml` manifest.

For instance,

```yaml
  elements:
    # List of images in the element
    - images:
      - name: genesis-base
        format: qcow2
```

It may looks like
```bash
genesis build --override elements[0].images[0].format=raw
```

The current solution is editing it in the code.