Update Snappy restart strategy

The snappy restart strategy was built on top of checkbox-cli2 and is expecting each snap to have a special checkbox-autostart file calling checkbox-cli2.
Now checkbox-cli does offer the same feature since its full rewrite (to use session assistant).

Updates required:

1. It's time to kill checkbox-cli2 and only require snap wanting to restart session on snappy automatically on boot to just have a checkbox bin using checkbox-cli.

2. I've checked the plainbox strategy code and it needs to be updated since

        self.config.set('Service', 'ExecStart',
                        '/apps/bin/{}.checkbox-autostart --resume {}'.format(

should be

        self.config.set('Service', 'ExecStart',
                        '/snap/bin/{}.checkbox-autostart --resume {}'.format(

on series 16