Convert Subnet to OVO in ipam_pluggable_backend.py
Convert Subnet to OVO in ipam_pluggable_backend.py

Change-Id: Ie3531ce47cff6efeb8a14a974e6587e63740148b
Partial-Implements: blueprint adopt-oslo-versioned-objects-for-db

openstack/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,mahak/neutron