Use Python 2.7's check_output in tails-additional-software

The tails-additional-software script reads: “XXX: replace with check_output when Tails will be based on Wheezy”. Presumably, the 3 lines that follow were copied straight from Python 2.7’s implementation of check_output.

The code lives is there: https://git-tails.immerda.ch/tails/tree/config/chroot_local-includes/usr/local/sbin/tails-additional-software

Related issues

Blocked by #6015 (closed)

Original created by @intrigeri on 7041 (Redmine)

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