Fix _build_instance_node for building Compute CDM

As of Nova API microversion 2.47, response of GET /servers/detail has flavor
which contains a subset of the actual flavor information used to create the
server instance, represented as a nested dictionary.

Since current watcher nova default api version is 2.53(Pike), This patch
follows the API response change.

Change-Id: Ia575950f0702afa1d093f03ca8ddedd3c410b7de
Closes-Bug: #1722462

watcher/tests/decision_engine/cluster/test_nova_cdmc.py

@@ -19,7 +19,6 @@
 import mock
 
 from watcher.common import nova_helper
-from watcher.common import utils
 from watcher.decision_engine.model.collector import nova
 from watcher.tests import base
 from watcher.tests import conf_fixture
@@ -62,9 +61,6 @@ class TestNovaClusterDataModelCollector(base.TestCase):
         # m_nova_helper.get_instances_by_node.return_value = [fake_instance]
         m_nova_helper.get_instance_list.return_value = [fake_instance]
 
-        m_nova_helper.get_flavor.return_value = utils.Struct(**{
-            'ram': 333, 'disk': 222, 'vcpus': 4})
-
         m_config = mock.Mock()
         m_osc = mock.Mock()