Make resource tracker use conductor for listing compute nodes

This makes the resource tracker use the existing conductor method
of service_get_all_compute_by_host() instead of querying the
database directly.

Related to blueprint no-db-compute

Change-Id: Iee19eeaa8651e4df0abb974f3f3ddeafd9d23409

nova/compute/resource_tracker.py

@@ -309,7 +309,8 @@ class ResourceTracker(object):
 
     def _get_service(self, context):
         try:
-            return db.service_get_by_compute_host(context, self.host)
+            return self.conductor_api.service_get_by_compute_host(context,
+                                                                  self.host)
         except exception.NotFound:
             LOG.warn(_("No service record for host %s"), self.host)