This should be completely backwards compatible. It allows the '-f' part
of the nix-env command to be configured. This greatly eases using
nixos-container as part of development where several nixpkgs
repositories might be tested at the same time.
Using 'machinectl kill' is much faster then gracefully stopping the
container.
In the case of 'destroy', since we're destroying it anyway, there's no
reason to do a graceful shutdown.
This moves nixos-containers into its own package so that it can be
relied upon by other packages/systems. This should make development
using dynamic containers much easier.
