traffic_replay: Convert print() to logger.info()

traffic_replay: Convert print() to logger.info()

Using logger is more helpful here because it includes timestamps, so we
can see how long things are taking. It's also more consistent with the
rest of the traffic_replay logging.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>