adding documentation about dest-node in psconfig

Hi Guys

One useful feature that Andy show me was the ability to set the dest-node in the psConfig mesh configuration. I couldn't find any documentation on it. The use case specifically for this is if you are trying to use multiple linux network name spaces.

Specifically in this example look at the "pscheduler-address"

      "address": "Foo",
      "contexts": [
        "linuxnns_i2px"
      ],
      "pscheduler-address": "Bar",
      "_meta": {
        "display-name": "I2 Cloud Clev I2PX",
        "display-url": "https://Bar/toolkit/"
      }
    }```

Then you would use in your test spec the pscheduler address like below

```  "tests": {
    "throutput": {
      "type": "throughput",
      "spec": {
        "duration": "PT30S",
        "source": "{% address[0] %}",
        "dest": "{% address[1] %}",
        "dest-node": "{% pscheduler_address[1] %}",
        "omit": "PT5S",
        "ip-version": 4
      }
    }
  }```

Start by locating the existing psconfig mesh-configuration documentation and the sections covering the "pscheduler-address" and test-spec fields. Document how "dest-node" supports Linux network namespaces using the provided Foo/Bar configuration and throughput example; done means a reader can understand and reproduce this setup.