Fixes #5633: Enable systemd support in ncf

Ticket: http://www.rudder-project.org/redmine/issues/5633

To be merged in master
Given the implementation below, I don't think that this restriction really is based on Linux vs non Linux systems. Your impregnation is smart enough to detect if it has the required tools and use the most advanced one available (systemctl over service over init.d). This is great. But therefore you should follow through with this logic, and check if the init script exists. Then, if and only if it doesn't, report an error. 

This way we will be much more inclusive of systems that have working init scripts (BSD by default, also possible on AIX, etc) and systems that just don't have a Linux kernel.