To support network hotplugging, don't auto-delete router-interface ports.

When a router is unplugged from every subnet, the current implementation
deletes the port, which makes it impossible for Nova to remove the vif, leaving
an orphaned interface attached to the router instance.  Instead, let orphaned
router-interface ports hang around so that they can be detected, detached,
deleted by the rug during the hotplugging process.