Setting up variant system services

oneSIS only needs to handle services that vary between nodes. If a service runs on every node, it should be handled through the distribution's normal mechanisms, for instance chkconfig on RedHat systems.

When a service should only run on some subset of nodes using the image, SERVICE directives can be used to specify which services should run on which nodes. The mk-sysimage script must be run after adding or removing a SERVICE directive.