The group name of cert related options have been changed from
keystone_authtoken to keystone_auth since Ocata[1].
This patch is aim to update them to avoid deprecated warning from oslo.config
[1] https://github.com/openstack/magnum/blob/master/magnum/conf/keystone.py#L19
Change-Id: Ic86eda5ca936e84da9a813aee8b6ceaa1c90eff7