Import oslo.config configuration file generator

This is stolen from Nova with a fix stolen from
https://review.openstack.org/#/c/30251/

Obviously the final goal is to stop copying this file into each project and
start improving it, to move it finally in oslo.config.

Change-Id: Ie81a5c3db1dc6092bcc6179ab0ab9499239f0bbc
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Import oslo.config configuration file generator"