nixos/networking: Use type lines for localCommands

Using types.str doesn't work if you want to mkBefore/mkAfter across
different module definitions, because it only allows for one definition
for the same priority.

This is especially useful if you deploy Hetzner machines via NixOps,
because the physical specification already defines localCommands.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>