<property name="LootDropEntityClass" value="EntityLootContainerBoss"/> <!--this is the actual loot container that they drop - create you own if you need-->
<property name="LootDropProb" value="0.5"/> <!-- the probability of dropping the above loot bog, in this case 50%-->
I dont know if there is an overall drop probability variable - the above is for each entity.
Edit: You can set a global loot drop probability of individual items but probably not lootbags.