hetzner: Trim bootstrapper if tmpfs is too small.

Before pushing the bootstrap tarball onto the rescue system, we now
check whether there is enough free space in the root filesystem and if
not, we'll try to get down the size bootstrap package as hard as
possible.

This is only necessary on the smallest vServer (VQ7), but who knows
whether people still have servers below 1 GB of RAM.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix/hetzner: Add example for vServer partitioning.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Merge pull request #164 from aszlig/hetzner-vserver

Add support for Hetzner vServers.