Delete duplicate external devices in router namespace

When a router's gateway is removed during L3-agent restart, the agent
will fail to delete the old external device.  This device should be
identified and removed as soon as possible.

Change-Id: Ifd3e6ca009242138bea4a098e3fde258aeaa391e
Closes-Bug: #1298658
Merge "Delete duplicate external devices in router namespace"