Always enable container logins
container-login.nix -> container-config.nix
Make starting a container synchronous

So now "systemctl start container@foo" will only return after the
container has reached multi-user.target.
Disable container support in containers

Systemd-nspawn doesn't support nesting, so providing nixos-container
inside a container doesn't make sense.
nixos-container: Don't destroy declarative containers
Add a test for NixOS containers