As for config options we are using oslo_config library.
It limits us to override config options based on used deployment/env.
To get control of all options and how they are obtained, let's restrict
usage of oslo_config directly and force everyone to use rally.common.cfg
which has the same interface.
Change-Id: Ia845958b50b62d971f1343c03da2552049aa4363
f94d3efc76