Add support for pluggable l3 backends
Add support for pluggable l3 backends

This will allow us to support backends other than linux_net (i.e.
quantum L3 when it is available) for defining L3 connectivity.

Change-Id: I0b2ece2278bd68166741107a88cedd106d1ab651

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