(FACT-618) Cherry-pick lost zpool_version fix into master

This commit cherry-picks work done on the zpool_version fact,
which was lost during the facter-2 / master merge.

Original commit (363191011):
(#20617) add support for zfsonlinux + fixtures

"zpool upgrade -v" has a "FEAT DESCRIPTION" section that other ZFS
implementations don't have. The regex in the zpool_version fact choked
on this difference. It was replaced by same one used in the zfs_version
fact.

Thanks to Trey Dockendorf for his helpful bugreport!