Fix obj_load() in NovaObject base class

The obj_load_attr() method is correctly used in the field getter,
but incorrectly provided in the base class (without the _attr
suffix). Rename that and add a test for the base class that makes
sure this method gets poked appropriately.

Related to blueprint unified-object-model

Change-Id: I3b010ef727fed2f59fbb3d7173078b1dbc4b81f6
Merge "Fix obj_load() in NovaObject base class"