use/bandwidth-usage.tree: Added $FIRST_EXECUTION test(1)

If it's the first time execution of the agent, do not collect network data
and exit without error.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Klismann Smoger Mottin <ksm09@inf.ufpr.br>
Signed-off-by: Josiney de Souza <josineys@c3sl.ufpr.br>