Fix empty association attributes

If an association does not exist, the id methods affected
in this patch will currently fail, as it will attempt to
grab an id of a non-existent attribute.

The implemented fix uses the EAFP philosophy.

Change-Id: I97b608bc267963e754d06379d94372d13e7c93fa

tuskar_ui/api.py

@@ -293,9 +293,9 @@ class Node(StringIdAPIResourceWrapper):
 
     @property
     def rack_id(self):
-        if self._apiresource.rack:
+        try:
             return unicode(self._apiresource.rack['id'])
-        else:
+        except AttributeError:
             return None
 
     @property
@@ -454,9 +454,10 @@ class Rack(StringIdAPIResourceWrapper):
 
     @property
     def resource_class_id(self):
-        rclass = self._apiresource.resource_class
-        resource_class_id = rclass['id'] if rclass else None
-        return resource_class_id
+        try:
+            return self._apiresource.resource_class['id']
+        except AttributeError:
+            return None
 
     @property
     def resource_class(self):

tuskar_ui/test/test_data/tuskar_data.py

@@ -123,7 +123,7 @@ def data(TEST):
                               {"name": "total_memory",
                                "value": "2",
                                "unit": "MB"}],
-                         'resource_class': None})
+                         })
     TEST.tuskarclient_racks.add(rack_1, rack_2, rack_3)
     TEST.tuskar_racks.add(api.Rack(rack_1), api.Rack(rack_2), api.Rack(rack_3))