Support OpenRC for Alpine packages

The Alpine distro usually doesn't provide a component to manage services by default. Users are responsible for setting it up on their own if they need it. They can choose systemd, but it is often too heavy for Alpine-related purposes.

The lightweight alternative is OpenRC. Stork could provide the service file in OpenRC format (as we do for systemd) and call the OpenRC manager (rc-service) commands in the package hooks (we do it for systemd).