Adds a NodeTrait model to the DB and DBAPI methods for the following:
* Set/unset node traits
* Get node traits
* Add/remove a single trait
* Check for existence of a node trait
All node traits are deleted when their owner node is deleted. Traits
cannot be specified on node creation currently.
Change-Id: I8b375a33dcb94d2cc73eaa93a5cc5aed86c27178
Partial-Bug: #1722194