fix dvr snat bindings for external-gw-clear

fix dvr snat bindings for external-gw-clear

When router_gateway_clear happens, the
schedule_router calls the unbind_snat_servicenode
in the plugin. This will clear the agent binding
from the binding table. But the l3-agent was
expecting the ex_gw_port binding to be present.
The agent needs to check its cache of the
router['gw_host_port'] value now.

Change-Id: I051fb97d802b0508b30683a33673b85f5ab24000
Closes-bug: #1373524