Stop using generator._list_opts; use cfg.CONF instead

We don't use the information about defining module anywhere. Using
that routine just makes the code slower and causes crashes in Neutron
and possibly elsewhere whenever an option is defined in the top level
but not registered.

Change-Id: I946c740cd0ce6fa5d969964d79a75a406e8453ab
Merge "Stop using generator._list_opts; use cfg.CONF instead"