Ability to filter which configuration options are available to a given module

At the moment, if a module requires a configuration option from another module, we do:

  CONF.import_opt('source.module', 'option_name')

but, in fact, all options from the imported module are available for use.

An alternative would be to enforce which options we available within a module e.g.

  CONF = cfg.FilterView(cfg.CONF, 'option_name')