Back2Back: update default params

Back2Back: update default params

Update the default duration of a back to back test to be 2 seconds which
is the minimum recommended by RFC2544, and update the Trials to be 50,
which is also the default value specified by the RFC. This patch also
fixes the tcl script to allow for the minimum framesize to be sent from
the traffic gen.

JIRA:

Change-Id: I801196c1998ec2bffbeab69a3a290d6ef6feb1c3
Signed-off-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Billy O Mahony <billy.o.mahony@intel.com>