Implement a piuparts task

As a debusine user, I can run piuparts on binary packages of my choices.

Design concerns

We should use this case to implement a generic "run this command, show the output and display an error/result".
[Already done through TaskRunCommandMixin / !327 (merged)]

Progress

• Design the interface for the piuparts task (almost no options expected)
  • Basic interface specified (!336 (merged))
  • Review/rework interface based on real-world scenarios (more options? N invocations for each .deb vs. 1 invocation for all .deb-s at once? deal with conflicting packages semi-automatically?...)
• Implement the Task class in Python
  • Basic implementation based on --schroot (!336 (merged))
  • Review/rework implementation based on #189 (closed) outcome (along with sbuild and autopkgtest tasks)
• Implement the ansible role to setup the worker
  • sudo-based implementation, similar to piuparts.debian.org (freexian-team/ansible/debian-infrastructure!9 (merged))
  • Review/rework implementation based on #249 (closed) outcome
  • Make ansible role more generic / less debusine-centric?