Collect network usage data from agent and make it stoppable when overflow

We're relying on the grinder statistics to make network safe.
Currently, agent network usage data is not collected.
However it's not enough because if a user run a test which does not use HTTP Plugin, the network usage is not collected.
Make the agent controller collect the network usage using Sigar.

Then collect network usage data from agents from a given region. If it's over the given limit, stop all tests in the region.