bugfix: l3 api-def router constant
bugfix: l3 api-def router constant

[1] introduced the l3 api-def into neutron-lib and
along with it a bug; the ROUTER constant has a value
of 'routers'. This patch fixes the issue.

[1] https://review.openstack.org/411974/

Change-Id: I4bb3843e7bf3d6b5c4e666b6e34d0f254428f40b

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib