HA Docs: Document Controller Nodes in Active / Active setup

This is the first commit of active / active section for Controller nodes
in the OpenStack HA guide.

bug #1196098
Blueprint improve-high-availability-support

Change-Id: Iaabbf64b425ccff810cc6a6656b6d018254e865a
OpenStack-HA: Fix list issue

Change-Id: I6550fdad1e87e03bbebef1f00ed00795bbd21c58