Handle refTable values with setkey()

Handle refTable values with setkey()

For columns like QoS.queues where we have a map containing refTable
values, assigning w/ __setattr__ e.g. qos.queues={1: $queue_row}
works, but using using qos.setkey('queues', 1, $queue_row) results
in an Exception. The opdat argument can essentially just be the
JSON representation of the map column instead of trying to build
it.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>