'Register the options in the shared group.
When we go to get a config option we will try the backend specific
group first and fall back to the shared group. We override the default
from all the config options for the backend group so we can know if it
was set or not.'

def _ensure_config_values(self, volume_opts):

    for opt in volume_opts:
        self._safe_register(opt, SHARED_CONF_GROUP)
        if (self.config_group != SHARED_CONF_GROUP):
            self._safe_register(opt, self.config_group)
            CONF.set_default(opt.name, None, group=self.config_group)
