Refactor the way of endowing the accounts in chain_sepc genesis functions

[ayushmishra2005]

also, FYI, this might be a cleaner way of endowing the accounts than the way we've done it https://github.com/AcalaNetwork/Acala/blob/master/node/service/src/chain_spec/karura.rs#L101

Originally posted by @ltfschoen in #217 (comment)