Use oslo.config directly instead of common.config

We've been using a wrapper around oslo.config that worked well at the
early stages of the project. Now, as the project grows, we need to user
more features from oslo.config that wouldn't make sense to export
through the wrapper we have.

This patch deprecates marconi.common.config and uses oslo.config
directly where needed.

This is the first patch around the bp remove-global-config, upcoming
patches will completely remove the usage of a global instance.

Partially-implements remove-global-config

Change-Id: Ie14c6bae8cdb72a6ed93a12c34cd8b156e059872