Use unique binding_index for RouterL3AgentBinding
Use unique binding_index for RouterL3AgentBinding

This is because (router_id, binding_index) tuple is expected to be
unique, as per db model.

Closes-Bug: #1674434
Change-Id: I64fcee88f2ac942e6fa173644fbfb7655ea6041b

eayunstack/neutron,mahak/neutron,openstack/neutron,noironetworks/neutron,mahak/neutron,mahak/neutron,openstack/neutron,huntxu/neutron,openstack/neutron,huntxu/neutron,eayunstack/neutron,noironetworks/neutron