Support autopkgtest-virt-unshare in autopkgtests

This adds the infrastructure for:

1. Downloading and caching system-tarballs.
2. A common executor interface that can be used by all the tasks.

And specifically:

1. Implements an unshare execution environment. But not the instance part of the interface yet, as it isn't needed for autopkgtest.
2. Migrates autopkgtest to this common interface.
3. Changes the default autopkgtest backend to unshare (as that's the only supported backend right now).

Some technical debt is introduced:

1. There is no configuration for workers available to tasks (#239), so the system image cache is just stored in the worker's home.
2. There is no cleanup of old downloaded images.

Part of: #223 (closed)

Fixes: #228 (closed), #219 (closed)