Shep Rollback

[chris-olszewski]

General idea Provide an easy way to revert an alias to a previous version and effectively "undo" a deploy
Implementation When enabled the user can specify in their config how many versions they wish to keep via rollback: n. This would create the following aliases for each environment ${env}-rollback-1, ... ,${env}-rollback-n. A rollback will just move the ${env} alias to the latest non-equal rollback alias.