Cleanup stdout/stderr output in general

[dlorenc]

• stdout should have machine parseable output
• stderr should have logs (when enabled) and user visible status output
• return codes should be 0 unless there is an error
• We should let users customize the output to stdout using a go template string

[dlorenc]

cc @sharifelgamal

[tejal29]

Integrate with CTC and this will be done. Here is a sample PR. https://github.com/GoogleContainerTools/container-structure-test/commit/9c37b86154503dfd842056c75b36a9bab6407b5b