Allow remote ZFS workspace allocator to snapshot after successful build

[hach-que]

The remote ZFS server should be able to accept a "snapshot" request on the workspace, which UET should send it after the build is successful. Further uses of the template should clone from that new snapshot for a duration (either clean it up when the ZFS snapshot server exits or some maximum duration like 24 hours). This allows subsequent builds to re-use the existing script initialization and not take as long.