Manually fix a wrong default in the config ref

Add the missing /v2 at the end of the neutron nova_url option.

The tools and dependencies brought by pip have changed since the
Icehouse release so running autohelp.py generates a lot of changes that
we don't want to see happening.

Change-Id: Ibac14689f0ba855efb00008ecd74657641a365ce
Closes-Bug: #1338613