bugfix: Parameter duration does not work

bugfix: Parameter duration does not work

Vswitchperf help describes two different options, which will
set lenght of the test or duration of its iteration. It can be set
either by parameter --duration or by --test-params CLI option
duration. However only the second option is implemented. It is
enough to support only one option and thus --duration parameter
has been removed.

Change-Id: Icfa40ec25f50bd090229b0761fa968567d8d3553
JIRA: VSPERF-196
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>