Check resource class before using

check if the resource class(VCPU, memory, disk) in the return
dictionary. If they are, don't need to use dict.get() with a
default value because the parameters are required.

Partially Implements: blueprint improve-compute-data-model

Change-Id: Icb8c672d0e87e6e5f030a2222f928d1bbd069e3c

watcher/tests/decision_engine/cluster/test_nova_cdmc.py

@@ -332,7 +332,7 @@ class TestNovaModelBuilder(base.TestCase):
         """"""
         mock_placement = mock.Mock(name="placement_helper")
         mock_placement.get_inventories.return_value = dict()
-        mock_placement.get_usages_for_resource_provider.return_value = dict()
+        mock_placement.get_usages_for_resource_provider.return_value = None
         m_placement_helper.return_value = mock_placement
         m_nova.return_value.get_aggregate_list.return_value = \
             [mock.Mock(id=1, name='example'),