Address config incompatibility introduced with oslo.config >= 1.2.0

This patch changes how nvp cluster specific configuration is retrieved
from nvp.ini so that it no longer relies on _cparser, which has been
removed in newer versions of oslo.config.

Change-Id: Id2223b4bea3a290ba9374ffdde264c96c502249f
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>