Running tempest failed when multi identity backends are configured

[Tao Li]

If we want to use multi identity backend, such as SQL and LDAP together in one openstack。so we should modify the keystone configration file as follows。

[identity]
domain_specific_drivers_enabled = True
domain_config_dir = /etc/keystone/domains

when "domain_specific_drivers_enabled" is set true. The list_groups,list_groups,list_users_with_name operations in tempest will get the groups,users scoped the domain owned the current user, so we can't get other domain's groups,users.

hence, we should add a configration item "domain_specific_drivers_enabled" to identity this scenario。