Track provider traits in report client

With this change, SchedulerReportClient keeps track of resource provider
traits in the same way as aggregates.  Specifically:

- We use ProviderTree methods to save trait information for each
  provider.
- We refresh trait information along with aggregate information whenever
  we "ensure" a provider exists.

Note that this change set consolidates the refreshing (including cache
expiry) of aggregate and trait associations into a single method.  The
theory being that both aggregate associations and trait associations
should change out of band with roughly the same frequency (i.e. very
rarely).

Change-Id: I33d01d8cab43ce6cdde151d2e9429921a140e88d

nova/tests/functional/api/openstack/placement/test_report_client.py

@@ -116,13 +116,16 @@ class SchedulerReportClientTests(test.TestCase):
             rps = self.client._get_providers_in_tree(self.compute_uuid)
             self.assertEqual(1, len(rps))
 
-            # We should also have an empty list set of aggregate UUID
+            # We should also have empty sets of aggregate and trait
             # associations
             self.assertEqual(
                 [], self.client._get_providers_in_aggregates([uuids.agg]))
             self.assertFalse(
                 self.client._provider_tree.have_aggregates_changed(
                     self.compute_uuid, []))
+            self.assertFalse(
+                self.client._provider_tree.have_traits_changed(
+                    self.compute_uuid, []))
 
             # TODO(cdent): change this to use the methods built in
             # to the report client to retrieve inventory?