pkt_gen: change duration to int for Dummy pkt_gen

pkt_gen: change duration to int for Dummy pkt_gen

In case there is specified concrete value for "duration" in the
configuration there is raised TypeError exception
in the send_rfc2544_throughput function of dummy traffic generator.
Conversion to integer solved this issue.

Change-Id: Ib37ecabc1000369d807a997c88d7a3d7aca425d5
Signed-off-by: Robert Wojciechowicz <robertx.wojciechowicz@intel.com>
Reviewed-by: Dino Simeon Madarang <dino.simeonx.madarang@intel.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>