behave_tests: tweak nfvbench results JSON filename

In case nfvbench packet rate is expressed as a percentage of the max throughput
rather than a value in pps or bps, use that percentage to build nfvbench
results JSON filename.

This is sometimes needed for results post-processing because the percentage
information cannot be found in nfvbench results itself.

Change-Id: I7d16dba16a733a8ee58a6f80ce4df40cb40e9843
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>