treewide: set defaultText for options using simple path defaults

adds defaultText for all options that set their default to a path expression
using the ubiquitous `cfg` shortcut bindings.

nixos/modules/services/computing/slurm/slurm.nix

@@ -1,10 +1,11 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, options, pkgs, ... }:
 
 with lib;
 
 let
 
   cfg = config.services.slurm;
+  opt = options.services.slurm;
   # configuration file can be generated by http://slurm.schedmd.com/configurator.html
 
   defaultUser = "slurm";
@@ -89,6 +90,7 @@ in
         storageUser = mkOption {
           type = types.str;
           default = cfg.user;
+          defaultText = literalExpression "config.${opt.user}";
           description = ''
             Database user name.
           '';
@@ -153,6 +155,7 @@ in
       controlAddr = mkOption {
         type = types.nullOr types.str;
         default = cfg.controlMachine;
+        defaultText = literalExpression "config.${opt.controlMachine}";
         example = null;
         description = ''
           Name that ControlMachine should be referred to in establishing a