Disable TownGenerator and plant spawner by default

Junk and plant spawning is disabled by default. It can be enabled via
dayz_townGenerator config variable and positions blacklist (for trader
cities, etc.).

The trader objects spawn code was consolidated to functions.

SQF/dayz_code/compile/client_plantSpawner.sqf

@@ -62,6 +62,7 @@ _insideloop = {
 };
 
 for "_i" from 1 to 2048 do {
+	if (!dayz_townGenerator or {toLower worldName != "chernarus"}) exitWith {};
 	_pos = _a call psrnd;
 	_pos call _insideloop;	
 	if (_plantcount >= dayz_maxGlobalPlants) exitWith {};