NixOS module for `cardano-node`

[aciceri]

We should re-use the upstream NixOS module as much as possible.

The convenient "extension" that allows to set values of a configuration file as NixOS options I was talking about is called freeform modules and it seems that the upstream module already partially implements something similar through extraNodeConfig.

Remember to check if the docs are generated accordingly.

This issue's scope is just to make the module work using values from the globals module (and possibly adding new options in that module if needed).

Feel free to split this issue into sub issues, in case please link them here as a to do list.

[brainrake]

fixed in #45