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.
To resolve unit test errors after clean cherrypick, the bridge is
added to the unplug function call in external_gateway_updated.
(cherry picked from commit