Create a guide for scaling out Heat API's

This documentation describes a guide for scaling out the ReST and
CFN API's based on a devstack installation. HAProxy is used as a load
balancer, redirecting calls from CLI to the several API's instances.

Change-Id: I6efe988ce970f2a3aa08f39050f85dc1621e6774
Closes-bug: #1072958
Merge "Create a guide for scaling out Heat API's"