Ticket/master/pup 3388 zpool mirrors

(PUP-3388) Fixed zpool Provider for the mirror attribute
Ticket associated with this pull request: https://tickets.puppetlabs.com/browse/PUP-3388

Without this patch applied, multiple drives with mirrors could not be added.
This is a problem because the resource specifies that it could handle these conditions.

With the 1 line of code that I removed, and the 2 lines of codes that I added
I was finally able to bring the resource up to the standards of what the
resource was intended to do.