Remove test_flags comments, add ignore_failure flag
    
    It's not really a good idea to have the comments that explain
    the test_flags in *every* test, because they can go stale and
    then we either have to live with them being old or update them
    all. Like, now. So let's just take 'em all out. There's always
    a reference in the openQA and os-autoinst docs, and those get
    updated faster.
    
    More importantly, add the new `ignore_failure` flag to relevant
    tests - all the tests that don't have the 'important' or
    'fatal' flag at present. Upstream killed the 'important' flag
    (making all tests 'important' by default), I got it replaced
    with the 'ignore_failure' flag, we now need to explicitly mark
    all modules we want the 'ignore_failure' behaviour for.