allow NO_ZFS by setting zpool to false or ''

These days, poudriere no longer requires ZFS, but the module currently does assume that ZFS is used.

To make the module usable on all systems, allow users to set the NO_ZFS option in poudriere.conf by setting `zpool` to false or the empty string.

The default value is left at the old setting to not cause confusion for current users.

README updated with an usage example.