tools: lint for use of space in template strings

There are over 70 files in the project using template strings and all of
them have followed the convention of no spaces in curly braces.

Good: `${foo}`

Not used: `${ foo }`

Since the project has adopted a convention, and ESLint has a rule to
enforce exactly this convention, enable the rule.

PR-URL: https://github.com/nodejs/node/pull/6591


Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>