config: add "format' function [GH-758]

[mitchellh]

Fixes #758

This adds a format function. Docs:

  * `format(format, args...)` - Formats a string according to the given
      format. The syntax for the format is standard `sprintf` syntax.
      Good documentation for the syntax can be [found here](http://golang.org/pkg/fmt/).
      Example to zero-prefix a count, used commonly for naming servers:
      `format("web-%03d", count.index+1)`.

[tkak]

+1

[mitchellh]

Gonna merge this @phinze, let me know if any objections.

[phinze]

Belated LGTM 😁

[ghost]

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