neutron-lib: use L3 constant from neutron-lib
neutron-lib: use L3 constant from neutron-lib

Ensure that the plugin uses the constant defined in neutron-lib.
L3_ROUTER_NAT will be dropped in favor of this. This change is in
https://review.openstack.org/#/c/394164/.

NeutronLibImpact
Partially-implements: blueprint neutron-lib

Change-Id: I6f569d8f914a92c27d97a2f82c8f693157222e16

stackforge/networking-arista,openstack/networking-arista,openstack/networking-arista