Handle router external filter in net list

This fix is the cherry pick of
https://github.com/Juniper/contrail-neutron-plugin/commit/308c1ec8482c032fdc80bea29012a99be709c45c

Change-Id: Ia434f9407c0f4d77baf40330a3f3ab21a6efcfd5
Merge "Handle router external filter in net list"