nixos/taskserver: Rename .pki options

We're now using .pki.server.* and .pki.ca.* so that it's entirely clear
what these keys/certificates are for. For example we had just .pki.key
before, which doesn't really tell very much about what it's for except
if you look at the option description.

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