[Open Match] Make number of players per session configurable

[markmandel]

This mostly came out of trying to do demos with 4 instances of the game running locally, and watching my machine fall over because too much Nvidia card usage.

It would be lovely if as a terraform variable you could set how many players you wanted in a match. This would likely make demos (and testing) much easier, as you could set it to 2 players quite easily, but then do group tests for as many as you like.