Adding a key value to 'choice' config

#### What's this PR do?
Serves the purpose of both flagging that it's required since end-level and end-game logic reference these values, and also hopefully clarifying its use in code. Previously, this value needed to be the first item in the `valid_answers` array.

#### What are the relevant tickets?
Eh, no tickets. This is just a small thing that came up and made sense to do.