Move to using ovo's remotable decorators

This removes Nova's remotable and remotable_classmethod decorators
and aliases the old location to the oslo.versionedobjects (ovo)
implementations.

This uncovered a couple other places where we were still passing a
context to a remotable method (in tests), which are fixed up here.

There is also a bug in ovo's remotable decorator that requires dict
compatibility. For this reason, this patch adds the Dict mixin to
a few objects (with appropriate comments), which can be removed when
we have an ovo release with that fix in it.

Related to blueprint use-oslo-objects

Change-Id: Id51fa3a55f65c8c075423f50a1240c997f9f2388

nova/objects/cell_mapping.py

@@ -10,6 +10,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from oslo_versionedobjects import base as ovo
+
 from nova.db.sqlalchemy import api as db_api
 from nova.db.sqlalchemy import api_models
 from nova import exception
@@ -17,8 +19,10 @@ from nova.objects import base
 from nova.objects import fields
 
 
+# NOTE(danms): Maintain Dict compatibility because of ovo bug 1474952
 @base.NovaObjectRegistry.register
-class CellMapping(base.NovaTimestampObject, base.NovaObject):
+class CellMapping(base.NovaTimestampObject, base.NovaObject,
+                  ovo.VersionedObjectDictCompat):
     # Version 1.0: Initial version
     VERSION = '1.0'