In working patch 251989, Michal Dulko points out that
all the OVOs should have OPTIONAL_FIELDS inside object.
This patch will fix the issue in OVO, CGSnapshot.
Now that the OPTIONAL_FIELDS is part of the class definition,
_from_db_object should no longer be a staticmethod,
but should be a classmethod.
Change-Id: I9603cc3839dfd6e1cf102a77aeed3e9edc661bb4