nixos/tomcat: configure default group and fix broken default package reference
nixos/tomcat: configure default group and fix broken default package reference

Without this fix, evaluating a NixOS configuration with Tomcat enabled and the
default settings results in the following evaluation error:

Failed assertions:
- users.users.tomcat.group is unset. This used to default to
nogroup, but this is unsafe. For example you can create a group
for this user with:
users.users.tomcat.group = "tomcat";
users.groups.tomcat = {};

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