Moving the OPTIONAL_FIELDS inside the OVO  object
Moving the OPTIONAL_FIELDS inside the OVO  object

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

Datera/cinder,mahak/cinder,Nexenta/cinder,cloudbase/cinder,phenoxim/cinder,eharney/cinder,bswartz/cinder,phenoxim/cinder,ge0rgi/cinder,openstack/cinder,cloudbase/cinder,NetApp/cinder,j-griffith/cinder,Hybrid-Cloud/cinder,NetApp/cinder,Nexenta/cinder,j-griffith/cinder,openstack/cinder,bswartz/cinder,mahak/cinder,eharney/cinder,Hybrid-Cloud/cinder,Datera/cinder