Propagate missing token count which differs for HRO only

This fix the previous merge request (!433 (merged)) where the configured value was not passed from hro-cloud/grid.nix to lib/issuer.nix!

The result is already live on HRO payment server: --tokens-per-voucher 150000 has been append to the parameters passed to zkapissuer service. And it has fixed the problem: vouchers can be redeemed because now both the client and the server agrees on the same predefined value.

This should close #148 (closed) for good.