CIDEVSA-499: Dicts in preload now support nested entries

Similar to DotDict syntax, the following input is now supported in "simple dict" entries in preload sheets (specifically intended for startup_config entries):


Input data:
```
grandparent.parent.child.attr1 = 3
grandparent.parent.child.attr2 = blue
```

Output data:
```python
{'grandparent': {'parent': {'child': {'attr1': 3, 'attr2': 'blue'}}}}
```