tls: remove util and calls to util.format

Currently util.format is being used for string templating in tls.
By replacing all of the instances of util.format with backtick
string we can remove the need to require util in tls all together.

PR-URL: https://github.com/nodejs/node/pull/3456


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>