Make relationships in objects consistent

Make relationships in objects consistent

In Volume object 1:n relationship on volume_attachment was modeled
by ListOfObjectsField. Moreover _from_db_object and obj_load_attr
actually were setting VolumeAttachmentList as value of that field,
which is wrong behavior.

In CGSnapshot similar relationship on snapshots was done by ObjectField
with SnapshotList inside.

This commit unifies the approach to use the latter. Also unit test is
added to prevent mismatch of field type and value set on it.

Change-Id: I802fc8807d7d4c42680bb19866c3e90c866d3f26
Closes-Bug: 1508889