generate documentation page about configuration options from the source

[Doug Hellmann]

It would be nice if we only had to describe configuration options in one place (code) and have that information reflected automatically in the doc build. We don't want to adopt oslo.config because that's overkill. Having a more detailed data structure in the config module, and then a sphinx extension that imports it and turns it into RST would be good enough.

[Doug Hellmann (doug-hellmann)]

The bug tracker for reno has moved to storyboard: https://storyboard.openstack.org/#!/project/933