Currently it's easy to create a new instack_env variable and forget to white-list it in 02-puppet-stack-config. This change makes instack_undercloud.undercloud module the primary source of truth on what instack_env can and cannot contain. Closes-Bug: #1660671 Change-Id: I82eef3ba4bb172b0260f72ce50f52bfb6bfc8e21