nixos/hbase: refactor settings option
nixos/hbase: refactor settings option

instead of keeping a defaultConfig value around, set that value as the
default of the option and explicitly use the option default instead.
this also allows us to write a defaultText that makes sense and is in
proximity to the definition of the default.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs