Add onionspray_use_systemd variable

This var allows to disable the Onionspray service, useful when running CI jobs in environments without Systemd.

See original PR here.

Opening a new one to suggest the name change: enabling a service in "systemd jargon" has a specific meaning (i.e. launching the service at boot), so I think using another name (like onionspray_use_systemd here) is less error-prone.

Also added a little commit for documentation. :)