Using a drop-in instead of an extra service avoids having to juggle
between both services (especially since irqbalance_mck did not have a
Conflict=irqbalance.service statement)
That way, we only have a single service to check for (irqbalance.service),
and system administrators should find this less confusing if they normally
rely on irqbalance.
The drop-in is also installed in /run so will automatically disappear in
the event of a linux crash or a reboot without shutting down mckernel
Change-Id: I004f4f25d9ca037e411e0bc91f4555db138ecfef
dfb3bef96d