Properly use obj_extra_fields in objects

Properly use obj_extra_fields in objects

The following patch fixes a bug in the snapshot object,
where fields that are not part of the Snapshot model,
i.e. name and volume_name, were not specified as
obj_extra_fields in the object. It also fixes the same
problem in the volume object, i.e. name and name_id.
With this patch, those extra fields are properly loaded,
e.g. snapshot.volume_name or volume.name.

Change-Id: Ie2b8dc66c897f89f941e9ad3b80ec4f4ab346430
Closes-Bug: #1429889