Add Delegate=yes to slurmd.service file to prevent systemd from interfering.

Without Delegate=yes, systemd will "fix" the cgroup hierarchies whenever
'systemctl daemon-reload' is called, which will then remove any
restrictions placed on memory or device access for a given job.

This is a problem especially since 'systemctl daemon-reload' may be called
automatically by rpm/yum or a variety of config file mangers, leading to
jobs escaping from slurmd/slurmstepd's control.

This setting should work for systemd versions >= 205.
https://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface/

Bug 5292.