Drastic reduction of "Free" ammo in ammo trucks

Ammo trucks were feeding artillery spam. I've reduced ammo trucks
starting "free" ammo to about 10 HE shells worth from before having 100
shells.  Because this ammo capacity is based off the cost of the ammo
used, the ammo truck should still be useful for smaller supplying
smaller vehicles and base defenses.

Cost to purchase a Arty rearm = 80000
Approx number of shells rearmed = 40
Cost per round = 200
Capacity of ammo truck = 10 rounds
New cost of ammo truck = 2000

So it's now the same price as rearming from the options menu.

Fixes #100