Add Network object
Add Network object

This adds a Network object to provide services and modeling
for nova-network and its use of the current SQLAlchemy Network
model.

Related to blueprint nova-network-objects

Change-Id: I7f943cda32fc98b3b03cdbb5d68e0c741053afaa

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects