ovs: add ofport_request option to ovs interface

When ovs interface connection is created a message with specification is sent to ovsdb. This message does not specify what port should ovs use so it is chosen randomly. When NM restarts and this connection is brought up again the port is reassigned to another random port. However this breaks some stuff so in this MR I introduce new option ofport_request which - when it is set - will tell ovsdb to use the port number given by value in ofport_request.