default to stderr or stdout for log/print messages

[dustymabe]

Carried forward from the conversation in https://github.com/coreos/coreos-installer/pull/212#discussion_r408668498

Basically should we default to printing messages to stdout or stderr? Discuss!

[lucab]

My personal suggestion is to expand @bgilbert and @cgwalters hints there and:

• switch all the eprintln to log statements, then have the logger setup where they should go (usual default is stderr)
• push up the few println in here and return datastructures instead, so that logic can accommodate other output formats/targets too
• this leaves us with the progress bar, for which I don't have good ideas other than a silencing flag (linked to the above)