test_spec: Expand deployment scenario in RFC2889 Broadcast Rate test

test_spec: Expand deployment scenario in RFC2889 Broadcast Rate test

In the RFC 2889 Broadcast rate testing, four test ports are required.
One of the ports is connected to the test device, so it can send
broadcast frames and listen for miss-routed frames.

JIRA: VSPERF-103

Change-Id: I5ee3d20c71d7debf6c7b09fc094f72b2d4c10684
Signed-off-by: Al Morton <acmorton@att.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Billy O'Mahony<billy.o.mahony@intel.com>
Reviewed-by: Gene Snider <eugene.snider@huawei.com>