Fix: Loading 'python' nested config

[timotheeguerin]

So the issue seems to have appear in release 5.0.0-preview.4 when m4 was updated. The change in m4 was the addition of prechecker which changed the pipeline loading order and broke this hack.

Change here is to forget about loading the python: nested configuration before modelerfour(This shouldn't be expected anyway) but just loading it before all the other plugins so it is available.

# This will work:
python: 
  output-folder: ./my-python-override

# But here modelerfour won't be able to use that  some-m4-config value
python: 
  modelerfour: 
    some-m4-config: foo

To achieve #2, you should be doing:

```yaml $(python)
modelerfour: 
  some-m4-config: foo
```

Using python: is meant to pass configuration only for the python plugin not if --python has been passed