Improve keystone.conf [credential] documentation
Improve keystone.conf [credential] documentation

Change-Id: Idefff0bbbb1c919a78ca92a3309d4d75ce4c31f0

cernops/keystone,mahak/keystone,openstack/keystone,ilay09/keystone,rajalokan/keystone,rajalokan/keystone,cernops/keystone,openstack/keystone,mahak/keystone,ilay09/keystone,openstack/keystone,mahak/keystone,ilay09/keystone,rajalokan/keystone