Sync the logic of _get_router_for_floatingip with Neutron
Sync the logic of _get_router_for_floatingip with Neutron

Neutron commit 526d28c5dba75e517bf833b895b69b840f03c6a4 changed
the logic. NEC plugin has its own _get_router_for_floatingip to
support OpenFlow router and the change breask NEC plugin.

Generally speaking, it is not a good idea to override a private
method, but there is no good interface exposed from L3 DB plugin
and it is a workaround.

Change-Id: I48f3957a083473a17cdabe100ff564e9b5370eec
Closes-Bug: #1532907

stackforge/networking-nec,openstack/networking-nec,openstack/networking-nec,stackforge/networking-nec