set bdm['volume_id'] to None rather than delete it

bdm is a sqlalchemy model object, it doesn't make sense to delete
an attribute, so just set it to None

Fix bug 1166160

Change-Id: I41017af602f0c8369d442b4a6936e1a829d2f9a9
Merge "set bdm['volume_id'] to None rather than delete it"