Make the MapFile a CVar (#2561)

Content.Shared/CCVars.cs

@@ -46,6 +46,9 @@ namespace Content.Shared
         public static readonly CVarDef<int>
             GameMaxCharacterSlots = CVarDef.Create("game.maxcharacterslots", 10, CVar.ARCHIVE | CVar.SERVERONLY);
 
+        public static readonly CVarDef<string>
+            GameMap = CVarDef.Create("game.map", "Maps/saltern.yml", CVar.SERVERONLY);
+
         /// <summary>
         ///     When enabled, guests will be assigned permanent UIDs and will have their preferences stored.
         /// </summary>