(CFACT-120) Remove zpools and zfs_datasets facts

We don't have a strong rationale for adding these facts, and they could have
a noticable impact on serialization performance and clutter the fact output.
Remove the zpools and zfs_datasets structured facts that enumerated properties
of each zfs zpool and dataset.
Merge pull request #179 from MikaelSmith/task/master/CFACT-120-remove-zpools-zfs_dataset-facts

(CFACT-120) Remove zpools and zfs_datasets facts