Network usage bw and 95th percentil

Hi,

This pull request change the bandwidth perf data format in order to return in bit per second in place of percent.

This allow us to proceed the 95th percentil. This is done in the graphite template.

I wasn't able to find any means in order to repeat a template for each interface returned by the plugin, so only the eth0 is templated but the template is easily editable.

Regards