Add init command

(Copied from the card for this feature.)

The host is initialized installing all the Launchpad dependencies and
the required tweaks, but no source code is retrieved. Create a new
subcommand in `lpsetup/subcommands`. Idempotent (nice to have). It
should be subdivided in steps: i.e use `StepsBasedSubcommand`. It
should reuse code already present in `lpsetup.subcommands.install`.

Optional arguments:
    -u --user: The name of the system user to be created or updated.
        The current user is used if this script is not run as root
        and this argument is omitted.
    -e --email: The email of the user, used for bzr whoami.
    -f --full-name: The full name of the user, used for bzr whoami.
    decision needed: --private-key, --public-key, --ssh-key-name?