ovs-benchmark: Improve output format for "rate" command.

ovs-benchmark: Improve output format for "rate" command.

Before, the "rate" command kept the status of the test continually updated
on the screen.  This form of output is nice to watch, but it isn't very
useful for reporting, because you only end up with a single number at the
end, and redirecting it to a file yields one long line with lots of
carriage returns.

This commit simplifies the output format by simply printing one line per
second.

Signed-off-by: Ben Pfaff <blp@nicira.com>