Use systemd drop-ins for temporary cgroup configuration

[fabiand]

Today the agent is directly configuring a cgroup, i.e.:

wasp-agent/contrib/agent.sh

Line 19 in c6bb6df

_set "$MAJMIN target=50" $FSROOT/sys/fs/cgroup/system.slice/io.latency

or

wasp-agent/contrib/agent.sh

Line 38 in c6bb6df

_set $MEM_HIGH $FSROOT/sys/fs/cgroup/kubepods.slice/memory.high

However, we can move to using systemd drop-ins in order to align better to how croi and systemd interact.
To also avoid that systemd might reconcile - overwrite - our configuration.

https://www.flatcar.org/docs/latest/setup/systemd/drop-in-units/