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.
Create baremetal nodes and register them in lockstep

It's pretty confusing that we create 3 baremetal nodes towards the
beginning of the instructions, add 1 to the seed cloud and then
later add the other 2 to the undercloud.

We can easily create the required number of new nodes and immediately
register them when they're needed.

This makes it a lot more clear what you need to do if you want to
recreate just the baremetal node for running the undercloud.

Change-Id: I9e669c83fb9403acf939458afa8fcf2d27d18f5e