nixos/logstash: update and simplify to be fully compatible with new version

New logstash config is not fully compatible with the way how this module used to generate config. New logstash config now supports conditional statements that can't be expressed with simple nix syntax, and even if we find a way to express it with nix syntax nobody is going to support this, based on how much logstash config syntax changes.

I simplified logstash config, by giving user an option to provide input, output and filter config as strings, nothing more fancy, but it works. I appriciate the original author who made an effort to convert nix syntax to logstash syntax, but this just does not work anymore.