Allow the service to start only after the package is installed

I was having an issue installing smartmontools on a Debian box, due to smartd.conf not being configured before the service was trying to start. Disabling smartd starting by setting ``` start_smartd=no ``` would work but ```shell_config``` overrides that setting before installing the package.

This PR ensures that the ```shell_config``` is called after the package is installed, but before the service is started.