mac-address/number.string: Use the getNetworkInterface() to obtain the nic

This garantees the script is going to obtain the network interface in use,
even in case of there is more than one default route(1).

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Signed-off-by: Josiney de Souza <josineys@c3sl.ufpr.br>