Be compatible with oslo.config 1.2.0a3+

The private API of oslo.config changed, and the Cisco
options parsing code was using it.
Use an explicit MultiConfigParser instance for parsing
the config files.

Fixes LP Bug #1196084

Change-Id: I7ffcac3c295491fe9ba8abc7e98f33157a48c51b
Merge "Be compatible with oslo.config 1.2.0a3+"