Fix typo in provisioner.js (lenth => length)
Fix typo in provisioner.js (lenth => length)

This has been causing there to be no limit on the number of
concurrent zfs_query tasks that provisioner can spawn, occasionally
leading to retry/timeout storms on busy servers with lots of
zfs datasets.

joyent/sdc-provisioner-agent,joyent/sdc-provisioner-agent