Supports get node by instance uuid in API

The API needs to expose a means to search for a node, given an
instance uuid. This patch supports getting a node by an instance
uuid (/nodes?instance_uuid=<xxx>), getting all the nodes that have
instance uuid associated (/nodes?associated=True)
and getting all the nodes that do not have such associations
(/nodes?associated=False).

Change-Id: I40750f9a7fc91e87223bfcd72e17e25f4045359c
Partial-Bug: #1239026