yang model revise: neutron-L3.yang

yang model revise: neutron-L3.yang

This patch revises yang model for neutron-L3 and related codes.
- remove backpointer as interface-attributes
- pyang warning. missing key in leaf list
- remove deprecated methods in neutron spi
  If openstack provider needs to find a list of interfaces attached
  to a given neutron router, it walk through a list of ports to
  find a port whose device owner is router and device id is uuid
  of the router.

Change-Id: I245a880437369448933e9a41feb078a8baecc4d9
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Ritu Sood <ritu.sood@intel.com>