API to pass fields to node object list

This change modifies the nodes _get_nodes_collection method to
consider and pass in an explicit lisst of requested fields into
the node list method, while also including the required fields
for things like ownership/policy checking.

And slightly modifies node_convert_with_links method to simplify
it while enabling field validity to be checked, and specific
requisite field lists provided in based upon that value.

And also optionally builds the traits list as they are nolonger
*always* populated on all objects with fully populated objects
as only partially hydrated objects are provided back when specific
fields are requested.

Story: 2008885
Task: 42572
Change-Id: Ided419263d84184cab902944b6c518f98618c9d2

ironic/tests/unit/objects/test_node.py

@@ -415,7 +415,6 @@ class TestNodeObject(db_base.DbTestCase, obj_utils.SchemasTestMixIn):
             self.assertEqual(self.fake_node['uuid'], nodes[0].uuid)
             self.assertEqual(self.fake_node['provision_state'],
                              nodes[0].provision_state)
-            self.assertIsInstance(nodes[0].traits, objects.TraitList)
             # Random assortment of fields which should not be present.
             for field in ['power_state', 'instance_info', 'resource_class',
                           'automated_clean', 'properties', 'driver', 'traits']: