Add Credential Authentication Usage Documentation
Add Credential Authentication Usage Documentation

This patch adds documentation on using the new credential
object. It is the last of several patches which will implement the
"Allow different Keystone Auth Support in Castellan" blueprint.

Change-Id: I22608e83f46b4cc77ad6a5e41a95389751b0ecbf
Implements: blueprint remove-keystone-dependency

openstack/castellan,openstack/castellan