Comment 10 for bug 1424900

Okay. Some of the thought above were wrong.
Here is what's happening.

Python3-based cloud-init on 15.04 reads binary data into UTF-8 string.
It thoughts that this bitstream is utf-8 but it doesn't.
If it meets sequence of bytes that can't be converted into single utf-8 character (some non-sense sequence) it puts 'unknown chracter' (\xef\xbf\xbd or �) instead.
Then, we want to right userdata string to a file we encode it into bitstream but get all this 'unknown character's encoded with \xef\xbf\xbd which makes output longer and add unexpected bits.
That breaks the whole thing.