Networking tests - Option to choose within and across AZ

[Deepthidharwar]

By default, pbench-agent is made to run on the first two worker nodes. Worker nodes can be on the same AZ or different and most often then not, one needs to manually re-assign, so that choose worker-nodes are across AZ. So it will be good to automate selection of worker nodes within or across AZ as part of network vars .