This adds a new feature flag on tempest conf whenever LDAP is enabled.
When this flag is set to True Tempest users and groups identity tests
adapt to fetch users and groups from different domains.
Change-Id: I368ddf34908b906355c422bd1afd6ab9b1a80053
Depends-On: Iedb470c51fa2174ab7651e6b7e22eff1f25f7aac