Import DB model base from neutron_lib
Import DB model base from neutron_lib

DB model_base previously in neutron.db, is now under neutron_lib.db.

Change-Id: I3cb854073a8f9a78efcf8a1599ee717579757b35

openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2