Reorganize api-ref: v3 regions-v3
Reorganize api-ref: v3 regions-v3

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

Change-Id: Ia536a36a6cab51c4b6911da1726281c196b54d22

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