Add two @properties to help facilitate use of single-node tasks,
and update docs accordingly.
While tasks should support multi-node operations, right now, most code
paths are fairly simple and only operating on a single node at a time.
The task.node and task.driver properties will make things DRYer.
Change-Id: Iff9202af623b1cdd25f6bc73bba679e47fa13a81