Make the per-grid inline module more explicit.

This is a first step in turning the arguments of the gridlib.customize-* modules into settings, so that they can be merged with the non customize- versions of the modules. (See #80 (closed)).

• Rename the inline module to grid-module (this should cause less confusion with nixos/modules/deployment.nix).
• Rename the config variable in each grid definition to grid-config. This lets the grid-module module take a config argument without shaddowing it.
• Move the ssh and deployment configuration into it[1] (from being haphazardly spread between nixos/modules, gridlib and gridlib.customize-).
• Make privateKeyPath and publicKeyPath into settings, rather than being passed as arguments. I also moved the source definitions for the ristretto and stripe secrets into the non-customize gridlib modules. The remaining secret sources can be moved when more of the arguments have been turned into settings.

[1] And argument could be made that this should be a shared module between the grids. I did not go with that solution here, since I'm working on creating a docker-based grid that *doesn't have ssh or deployment config.