Revert "zfs cannot be distributed. Disabling it in the isos."

ZFS's popularity is growing, and not including it by default is a
bit frustrating. On top of that, the base iso includes ZFS
_anyway_ due to other packages depending upon it.

I think we're in the clear to do this on the basis that Oracle
probably doesn't care, it is probably fine (the SFLC agrees) and
we're a small fish. If a copyright holder asks us to, we can
definitely revert it again.

This reverts commit 33d07c7e.