labs: split out vm_init_node

Move the initial creation and configuration of a node into a separate
function. Follow-up patches give the option to omit a call to this
function, which allows osbash to continue work on a node rather than
having to recreate it for each run.

Change-Id: Id58a5f572de0b6453cf24533b2b4e2337f35803f
Implements: blueprint openstack-training-labs