allow providing additional label to worker hosts

[snowch]

Currently, the worker hostname is set as follows:

${var.project_id}-instance-worker-${count.index + 1}

e.g. csnow-hcp-demo-instance-worker-1

It would be good if the user could provide an additional label to identify hosts, e.g.

${var.project_id}-instance-worker-${count.index + 1}-${var.worker_label[count.index]}

e.g. csnow-hcp-demo-instance-worker-1-mapr-fast-ingest

This could be achieved by an extra variable worker_labels, e.g.

worker_count = 4
worker_labels = [ "epic-host", "epic-host", "k8s-host", "mapr-fast-ingest" ]

This labelling would make it much easier to identify workers, e.g. when needing to ssh into a specific worker.