hetzner: Fix confirmation message on destroy().

The message should state "completely erase" if the wipe argument is
True, not the other way around.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests/hetzner: Work around failure on mkfs.btrfs.

When creating btrfs filesystems, the mkfs.btrfs tool tries to check loop
devices for existing mounts and fails to do so because it is unable to
find the mount point for the squashfs from the ISO.

Current mkfs.btrfs shouldn't have this problem, so it might be a good
idea in the long term to include filesystem utilities from <nixpkgs>.
Unfortunately, this will increase the size of the bootstrap tarball,
thus we keep the workaround until we have a way to remotely fetch the
tarball.

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