Generate traffic for a given amount of time

Sending prefefined amount of packets for each interface:vlan pair
can still result in random verify_networks failures on heavily loaded
environments

- sender will generate traffic based on time provided with --duration option
  or from config file, defaults to 20 sec
- repeat option will be used to configure amount of packets per iface:vlan
  pair sended in each iteration, defaults to 2 packets

Change-Id: Ie92a3ea175ca2ae9f43e79f66b449366a1b68126
Partial-Bug: 1306705