hetzner: Restructure generation of physical info.

This now encodes fs_info and hw_info with their surrounding braces
and/or the function head verbatim as they come from nixos-hardware-scan
and nixpart. The networking info now is encoded in JSON and thus the
machine state reflects this now properly.

The reason of this change is to not clutter up the physical spec any
more than necessary and the second reason is because we can now embed
the network info as the top-level of the backend's physical spec and
import fs_info and hw_info using nix2py.

Now, anyone who is using the Hetzner backend already needs to convert
his/her machine state to the new format. I've written a small update
script for that, which is available at:

https://gist.github.com/aszlig/6449773

Usage is something like this (assuming your current working directory is
a clone of the Gist):

nix-build && result/bin/nixops-update-hetzner-netinfo

Of course, if you already have sqlite3 in your PYTHONPATH you can also
directly run "python update.py".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix_expr: Remove parsing functionality of nix2py.

This should get rid of the parsing routines and the values fed into
nix2py are now just used as multiple raw values. The reason for this is
because it's actually not necessary anymore to merge, as we are now
using the module system.

Even though with the parser the pretty printed result looks really way
better than without:

https://gist.github.com/aszlig/850c88f54a479bc9866d

So, rest in peace little parser, as I don't think anyone else would
*demand* prettier output of the physical spec ;-)

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