Split up extracting Ironic driver_info method

The register_ironic_node method in nodes.py does a couple of things.
It does the extracting Ironic driver_info, and creating Ironic node.
So it is difficult to test a register_ironic_node method.
This fixes it.

Change-Id: Ib7edaeb6a75e9dde733bc01311b908b04c08365a
Merge "Split up extracting Ironic driver_info method"