[Merge] ~nteodosio/software-properties:deb822-apt-add into software-properties:ubuntu/master

[Nathan Teodosio]

> First is that I am not positive that by enabling deb822=True, we won't introduce regressions in other parts of the code that are not deb822-aware yet.

Sorry, I should have mentioned that I tested the other parts (and test_add_apt_repository.py also succeeded), but if I missed the multiple suites output in --list then that does puts the quality of my tests in doubt.

I went with your suggestions now, please have another look.
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https://code.launchpad.net/~nteodosio/software-properties/+git/software-properties/+merge/464417
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