netperf: introduce an agent to launch client sessions

Currently we use many threads/heavy control sessions to launch many
clients, Autotest takes too much host resouse.

This patch introduced an agent script, we execute it insider client
system to launch multiple sessions, and Autotest only need to use one
background thread to control clients.

We can consider to process the raw results in this agent script in
future, if it can help to simplify the case.

Signed-off-by: Amos Kong <akong@redhat.com>