MmDebstrap task

The mmdebstrap task fully implements the SystemBootstrap interface.

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

bootstrap_options
- use_signed_by (defaults to True): if set to False, then we do not pass the keyrings to APT via the Signed-By sources.list option, instead we rely on the --keyring command line parameter.

The task computes dynamic metadata as:

subject: suite name in the first bootstrap_repositories
parameter_summary: subject (architecture-variant)
runtime_context: $architecture:$variant:$extra_packages of bootstrap_options
configuration_context: architecture of bootstrap_options

The keys from bootstrap_options are mapped to command line options:

variant maps to --variant (and it supports more values than debootstrap, see its manual page)
extra_packages maps to --include

The keys from bootstrap_repositories are used to build a sources.list file that is then fed to mmdebstrap as input.