Add create() and destroy() to Node

What it says. Also make the code use them vs direct calls to dbapi. This
also removes passing context to node.refresh() and node.save() since
context is cached on the Node object on create.

Partial-Bug: 1314732

Change-Id: Id3fafe1eae70954095ed94dec86e4583b23c5af8
Merge "Add create() and destroy() to Node"