Do NOT hardcode interface speed for PROX tests

JIRA: YARDSTICK-1035

Do not hardcode NIC/interface speed in PROX test. Test assumes
NIC used is 10Gbps. This is incorrect. It could support 1Gbps,
10Gbps, 25Gbps, 40Gbps or something else.

This is used to calculate pps (Packets Per Second)
In Baremetal the NIC speed could be extracted. however when
run on a virtual machine this is not possible.

Solution:
Add in options section of test file.

eg.
Options:
  interface_speed_gbps: 10

Where 10 refers to a 10Gbps. In a setup where multiple interfaces
are used. This will refer to the speed of the slowest connection.

Change-Id: I89ab16479a2cdd1d79e52cbcc5a972762c60d057
Signed-off-by: Daniel Martin Buckley <daniel.m.buckley@intel.com>