On Ubuntu, provide an "ondemand.service" that replaces /etc/init.d/ondemand

The latter does not exist any more when "initscripts" falls out of the default
installation. This now does not do a fixed one-minute wait but uses "Type=idle"
instead. This also becomes a no-op when the CPU supports "intel_pstate" (≤ 5
years old), as on these the ondemand/powersave schedulers are actually
detrimental.

LP: #1584124
LP: #1579278