Document default value for tree_dn options
Document default value for tree_dn options

It wasn't obvious that if the *_tree_dn options were not set that
the value would be whatever the suffix option is set to.

Change-Id: I0246421083ae59e2684c10f1d6137ce8ef284f02

klmitch/keystone,dstanek/keystone,ilay09/keystone,himanshu-setia/keystone,rajalokan/keystone,jonnary/keystone,takeshineshiro/keystone,roopali8/keystone,jamielennox/keystone,rajalokan/keystone,ilay09/keystone,himanshu-setia/keystone,roopali8/keystone,mahak/keystone,openstack/keystone,rajalokan/keystone,dstanek/keystone,ajayaa/keystone,idjaw/keystone,mahak/keystone,maestro-hybrid-cloud/keystone,idjaw/keystone,dims/keystone,cernops/keystone,jonnary/keystone,openstack/keystone,klmitch/keystone,cernops/keystone,takeshineshiro/keystone,maestro-hybrid-cloud/keystone,dstanek/keystone,dims/keystone,openstack/keystone,mahak/keystone,jamielennox/keystone,ajayaa/keystone,ilay09/keystone