Fix pip install failure due to missing nvp.ini file

It looks like sdist does not support symlinks, therefore
letting nvp.ini point to nsx.ini is not a good solution.
Since nvp.ini is going away, leave a copy for now, but
add a warning so that users are aware of the switch,
whilst preserving full backward-compatibility.

Closes-bug: #1266555

Change-Id: Id98c41145712583072a8e63a5de31ecc1074e420