Do not create a swap zvol

Do not create a swap zvol

Using zvols as swap devices does create deadlocks for users, as of zfs 0.7.9.
As the discussion of the issue [0] shows, fixing this is involved, and probably
won't be happening anytime soon (before zfs 0.8).

Not creating swap as a zvol seems like the better default setting for now.

Users needing swap can either leave unpartitioned space, by setting minfree,
or create the zvol quite easily after installation

[0] https://github.com/zfsonlinux/zfs/issues/7734

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>