This reduces the confusion with the nixos/modules/deployment.nix module,
and should make it clearer that it can contain any configuration that
applies to all hosts on the grid.

Using AWS is what causes it so using AWS can be what fixes it, too.

... This breaks the tie, thx to https://github.com/DBCDK/morph/issues/146

Use convention to find the file from which the public key can be read.

I was about to make a new repository and complicate CI shell scripts to check
it out in the right place at the right time ... but why?