make creep upgrade a single purchase

[xxxLeviathanxxx]

the upgrade seem reasonable balanced for the most part. people who like the stronger tank abilitys get the armour, well what i see as too costly to put reall money into but the minelayers, and often tinkers upgrade the dmg. both have there reasonable traits but i speak as a tanker myself. i find that late game creeps die with no real purpose but to supply gold. i have found though that some player upgrade creeps get factorys and the advancded commander allowing them to spawn (with upgrades) quite powerful instant forces. but as i said unless they are used early game they appear to lose much use late game. even with armour upgrade players almost completely disregard creeps.