nixos/test/partition: Rewrite for nixpart 1.0.

Currently, this is still WIP and subject to change, but it helps to see
whether our storage configuration options actually work out the way we
want.

Still needs a lot of cleanup, especially regarding the -m option, where
I'm not sure whether we should do it with nixpart or write our own
lightweight solution to be built into NixOS.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nixos/tests: Enable "partition" as "storage".

Renames the test to closer match the NixOS module attribute and put it
into release.nix. Of course, those tests still fail, because nixpart is
still WIP and I haven't pushed the first version yet.

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