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.

Fixes bug #1200609

Change-Id: I631c6584ca6b45d0f716533bd1d711d049c6f157
Merge "Address config incompatibility introduced with oslo.config >= 1.2.0" into stable/grizzly
Merge branch 'stable/grizzly' of https://github.com/openstack/quantum into grizzly/stable