Tempest refers CONF.identity-feature-enabled.api_extensions for
getting Keystone configuration from tempest.conf, but the commit
I3a92397d7bc62c3ec156281696d5c22504a86cb3 adds unnecessary group
identity and Tempest cannot refer the configuration now.
This patch fixes it.
Change-Id: Id7e1a94b1078b5934d2d140e85594ac0010e2e73