labs: allow building individual nodes

So far, osbash can build either a basedisk or a cluster consisting of
three nodes (controller, compute, network).

With this patch, the user can choose to (re-)build only a single node.
For instance, "osbash.sh compute" will build only the compute node
while leaving controller and network nodes intact (if they exist).

Change-Id: I9c260cb600d7bfcba8f18252d8116c7473a10839
Implements: blueprint openstack-training-labs
Merge "labs: allow building individual nodes"