Portal - Custom RUSH/BREAKTROUGH is requred as possible gamemode

Main difference of `custom` is ability to set count of vehicles.

I've tried to create tank CO-OP mission to friday night battlefield, but once i choose rush/breaktrhough - i unable to set different count of vehicles available to the players.

More than that - AI player buy only one tank to fight vs players (i designed gameplay as small count of players with stronger tanks vs big count of AIs with weaker tanks). Ai just buy one tank and be destroyed, other AI players just waiting until that tank is available to get it again.