Change `encrypt stack` to a list of random, password, or empty

[trevordevore]

random uses uuid() to change password for every build. password is the original behavior where developer provides a password. An empty value will not encrypt stacks. true is equivalent to password for backwards compatibility.

Docs to update when merged into master:

• https://github.com/trevordevore/levure/wiki/app.yml#encrypt-stacks
• https://github.com/trevordevore/levure/wiki/How-do-I-use-password-protection-for-my-application-stacks%3F

Closes #133