Unfortunately spacemacs is not designed for installation
at use-package call time.
For use-package to be called a package must already be installed,
so I will continue on the initial plan, that is providing the
quelpa recipe at the <layer>-packages list level.
This is no bigdeal, this is basically moving :quelpa from
use-package to <layer>-packages variables.
Moreover it makes more sense to define package property at
declaration time instead of initialization (at least in spacemacs
world where installation is decoupled from configuration).