Make nova-network use Service object

There is a very small usage of service data in nova-network, which
can be converted to use the object method. Since it results in
the same database call as before, the tests need no change.

Related to blueprint nova-network-objects

Change-Id: Ie6ef37a3508c0ef3481d0217d3d085b855cfdc70
Merge "Make nova-network use Service object"