1. When dvr router is disabled and enabled back, the rfp
interfaces are deleted on the nodes without snat-* namespace.
This is due to creation of snat namespace during router
initialization stage on all the nodes. At later stages, since
the gw_port_host is not bounded on this node, external gateway
is removed which triggers removal of rfp interfaces and snat
Create snat namespace only on the nodes where gw_port_host is
2. In case of DVR SNAT, when the l3 agent is rescheduled to
another node, the rfp interfaces on qrouter-* namespace are
removed. Instead of calling external_gateway_removed() which
further deletes the rfp interfaces, the qg-, sg- interfaces
need to be unplugged and snat namespace need to be deleted.