objects: don't allow to update create_at and update_at
objects: don't allow to update create_at and update_at

Those fields are managed by sqlalchemy layer, we should not allow to
update them via OVO.

Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db
Change-Id: Ica8f50c45eb482e7a48d9b5b2b997cf9133b3100

eayunstack/neutron,eayunstack/neutron,huntxu/neutron,huntxu/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,openstack/neutron