Reorganize api-ref: v3 authenticate-v3
Reorganize api-ref: v3 authenticate-v3

Reorganize as per the documentation session at PTG:
https://etherpad.openstack.org/p/queens-ptg-keystone-doc-cleanup

Change-Id: I18177430489ce465c83b219b36cbbab65a3413ae

openstack/keystone,mahak/keystone,openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone