Modify the default latency from int to float if tbf is enabled.
Modify the default latency from int to float if tbf is enabled.

- the original version will caluse the latency to 0 when bandwidth
  is bigger than 200;
- the TC command doesn't allow use the 0.00ms as input for latency field

Signed-off-by: hwchiu <da3976304a8f716ced7038c8673b24fb3c0dc82a@gmail.com>

mininet/mininet,mininet/mininet,mininet/mininet