Add default port option for Factorio game server

[karlh5926]

• Add a default Factorio port option that can be specified via command line flag --game-port Or, via the the config json file with key "factorio_port".

• Add a conf.json option "factorio_port_lock". When set to true, force the game to run on the default "factorio_port" (--game-port on command line flag).

The default port will show up on the control page as the default value for game server.

In cases where where we need a factorio server port different from the default.

E.g. in my case, my admins can only use 34000 port since this is the only port that's forwarded. This feature eliminates the tedium & potential human error involved in having to maually edit the game port number everytime.