Implement a debootstrap task

The debootstrap task would implement the SystemBootstrap interface except that it only supports a single repository in the bootstrap_repositories key.

On top of the keys defined in that interface, it also supports the following additional keys in task_data:

• bootstrap_options

  • script: last parameter on debootstrap's command line

The various keys in the first entry of bootstrap_repositories are mapped to the corresponding command line options and parameters:

• mirror, suite and script map to positional command line parameters
• components maps to --components
• check_signature maps to --check-gpg or --no-check-gpg
• keyring_package maps to an extra package name in --include
• keyring maps to --keyring

The following keys from bootstrap_options are also mapped:

• variant maps to --variant
• extra_packages maps to --include

NB: This was part of an initial devel-blueprint design but since the mmdebstrap-based implementation is sufficient for now, this is left as a wishlist, it's not in any roadmap.