objects: forbid updates for project_id field for subnets
objects: forbid updates for project_id field for subnets

API forbids it, and there should probably be no use case to allow
updating the field from inside neutron.

Change-Id: Ie661ed11cf14edf328eb3f104ae410e4bf2e352d
Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db

cloudbase/neutron,huntxu/neutron,mahak/neutron,eayunstack/neutron,huntxu/neutron,openstack/neutron,igor-toga/local-snat,mahak/neutron,cloudbase/neutron,igor-toga/local-snat,noironetworks/neutron,openstack/neutron,eayunstack/neutron,sebrandon1/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,sebrandon1/neutron