bridge: Tolerate missing Port and Interface records for local port.

Until now, ovs-vswitchd has been unable to configure IP addresses and
routes for bridges whose Bridge records lack a Port and an Interface
record for the bridge's local port (e.g. OFPP_LOCAL, the port with the
same name as the bridge itself).  When such a bridge was reconfigured,
ovs-vswitchd would output a log message that worried people.

This commit fixes the internal limitation that led to the message being
printed.

Bug #5385.