Reorganize api-ref: v3 domains
Reorganize api-ref: v3 domains

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

Change-Id: I7180f8a6ca547041e1d5f33708341fb16cea0954
Signed-off-by: Suramya Shah <e41aafa9ca96be220bafbfb0cf36bbb893b8bc1b@gmail.com>

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