Move stemcells under meta + extract StufFile

This PR moves `stemcells` out of the to stub scope.
They can now be found under:
```yaml
meta:
  stemcell: # For when there is only 1 stemcell
    name: <stemcell_name>
    version: <stemcell_version>
  stemcells: # When there are multiple stemcells
    - name: <stemcell_name>
      version: <stemcell_version>
    - name: <stemcell_name>
      version: <stemcell_version>
```