Fix NSB NfVi support for 25 and 40Gbps

Fix NSB NfVi support for 25 and 40Gbps

When setting interface_speed option in the test case, NSB/PROX
was still considering 100% as 10 Gbps.
With this fix, when interface_speed is set to for instance 25 Gbps
then NSB/PROX will start generate at 25 Gbps.
Yardstick logging info printed on the screen will show performance result
as percentage of interface_speed (e.g. 100% will be 25 Gbps).
This is done by sending the command "speed 250" to PROX (PROX cannot
know the underlying interface speed in all cases e.g. in virtio, and
hence considers 100% as 10Gbps).

JIRA: YARDSTICK-1217

Change-Id: I4989161b04a7afa5e347935ee7ccef7dfe5a504d
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>
Signed-off-by: Emma Foley <emma.l.foley@intel.com>