Use oslo cfg module for tempest config.py

This changes how config.py parses and sets options. Instead of
using ConfigParser tempest.openstack.common.cfg is now used.

The configuration option usage remains the same so you access
options in the same way.

Change-Id: I6698f942dcc4cd917f3ea7ab8b489233dca633ce
Merge "Use oslo cfg module for tempest config.py"