Allow missing configuration values until build

[mosteo]

This way we can normally get/without crates without defaults, but at the time of building the values must be set.

Fixes #1125

[Fabien-Chouteau]

The patch looks ok but I am not sure about the solution used here.

In particular I don't like that the config files will be generated but only partially.
Could we skip entirely the config generation on the get step?

[mosteo]

Changed to avoid generating files in case of incompleteness. Also renamed a Map to Var_Map to be a bit more meaningful.