An instance property has been added to keep track of certain
modifiers that change the behavior of node requests. The modifiers,
delivered via config labels, are meant to track the AWS node
requests for spot instances or fleet.
Tracking these labels will provide better reporting of node
usage within Zuul jobs and the capability of jobs to react
on certain node types.
The node_properties property is expected to be accessible in Zuul
under the `nodepool` host facts.
Change-Id: I2d91f8d336738fc65ddaa26f3de6db5cba09ac0d
09b85516e5