Code review comment for lp:~paelzer/cloud-init/test-apt-source

Thanks for the repeated great review, due to that I changed:
- warning/errors in convert_to_new_format
- improved example wording

Further on I:
- extended test_convert_to_new_format to test wrong format
- fixed an issue identified the key* tests depending if it was already imported in gpg (that turned out to be a real issue with the implementation)
- rebased again to avoid merge conflicts

Will update commit message now ...