There are several parameters in [profile_name].Arma3AlphaProfile configuration file which is setting the default difficulty mode for the game. They are:
difficulty="veteran";
difficultyMP="veteran";

But if you will try to set the default difficulty for the Dedicated server using these parameters it would not work. When you logged as admin at the server, get the list of missions, choose the one you want to start and run it, it will be started using Regular difficulty only! There is no way to change it. This is very important on a public servers where users are able to vote for admin. It's easy to forget about setting the correct difficulty. Most of people even don't know where it is located.

Making this parameter working for Dedicated servers would be very useful for the most admins.