Allow create-nodes add to existing nodes

Interpret the $TOTAL arg as the number of new nodes to be created
and just add that number of new nodes if some already exist.

This allows us to create the seed baremetal node in one step and
then create the two undercloud baremetal nodes in another step.

Change-Id: Ie7b69584e348fbac24f09236c63ea42312ae5765
Merge "Allow create-nodes add to existing nodes"