Graph cluster model instead of mapping one

In this changeset, I use https://review.openstack.org/#/c/362730/
as an example to make the existing ModelRoot fully graph-based.

Change-Id: I3a1ec8674b885d75221035459233722c18972f67
Implements: blueprint graph-based-cluster-model

watcher/common/nova_helper.py

@@ -64,6 +64,15 @@ class NovaHelper(object):
             LOG.exception(exc)
             raise exception.ComputeNodeNotFound(name=node_hostname)
 
+    def get_instance_list(self):
+        return self.nova.servers.list(search_opts={'all_tenants': True})
+
+    def get_service(self, service_id):
+        return self.nova.services.find(id=service_id)
+
+    def get_flavor(self, flavor_id):
+        return self.nova.flavors.get(flavor_id)
+
     def get_aggregate_list(self):
         return self.nova.aggregates.list()