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/web-apps/discourse.nix

@@ -4,6 +4,7 @@ let
   json = pkgs.formats.json {};
 
   cfg = config.services.discourse;
+  opt = options.services.discourse;
 
   # Keep in sync with https://github.com/discourse/discourse_docker/blob/master/image/base/Dockerfile#L5
   upstreamPostgresqlVersion = lib.getVersion pkgs.postgresql_13;
@@ -399,6 +400,7 @@ in
           domain = lib.mkOption {
             type = lib.types.str;
             default = cfg.hostname;
+            defaultText = lib.literalExpression "config.${opt.hostname}";
             description = ''
               HELO domain to use for outgoing mail.
             '';