Improve creation of "report files"

[happz]

tmt saves several reports - check logs, phase results, etc. - and they should follow a standardized form. There were some differences, some reports did include timestamps, some did not, for example, report creators need to take care of appending new content, and so on.

• new helper, write_report(), to generate standard header.
• new Plugin.write_command_report() and Plugin.write_report() helpers, to generate standard reports on behalf of a phase. This is the interface plugins and checks use. There will be more plugins saving their reports - prepare/shell, prepare/artifact, etc. - and a single entrypoint will make things easier to maintain.
• improving documentation, hopefully.

Pull Request Checklist

• [x] implement the feature
• [x] write the documentation