Lazy evaluation of template variables

[ianbamforth]

Hi, I've been using this cookbook a while now, and wanted to wrap it in another cookbook in order to fetch my username and password from an S3 bucket. Turns out that I couldn't do this because template variables are evaluated at compile time, i.e. before my recipe has the chance to fetch the keys from S3. Any reason why the template variables couldn't be wrapped in laziness?

[phoolish]

I'm always open to PR's 😃