Mutibench multiplexing enhancement

[HughNhan]

Currently multiplexing a mutlbench run with uperf and iperf is not working. We may decide that multibench does not support multi iteration in which case this issue can be used as a documentation point.

Symptom:

The iperf client would fail on the second iteration when it could not determine the server IP. The endpoint-start-end shows:

{
  "svc": {
    "ip": "30042",  <======  BAD
    "ports": [
      30043
    ]
  }
}

On the first iteration, the good message looks like this:

{
  "svc": {
    "ip": "172.30.177.50",
    "ports": [
      30042,
      30043
    ]
  }
} 

Config:

A run iteration corresponds to a set of mv-params. To create a run with a two iterations, just add one more value to a list variable. For example:

{ "arg": "wsize", "vals": ["512", "1024"], "id": "1" },

Root cause:

Debugging shows that the iperf-server did not build the message correctly because the command args/opts passed to the iperf-server was not being multiplexed correctly.

First iteration passed to iperf-server-start with "opts= --ifname 'eth0' --ipv '4' --"
Second iteration passed to iperf-server-start with "opts= --"
W/o seeing --ifname, iperf-server-start will not look for IP.