[Enhancement]: Make slurmrestd deb package create slurmrestd system user rather than slurm_ops

[NucciTheBoss]

Per the slurmrestd manpage, the slurmrestd daemon cannot run as root or SlurmUser. This isn't an issue for the Slurm snap as we can create a dedicated user in the install hook, but this a problem for the Slurm Debian package as it does not create a dedicated user for slurmrestd; it only creates a slurm system user.

We can circumvent this issue by creating the user ourselves with slurm_ops using the following block of code:

try:
    subprocess.check_output(["groupadd", "--gid", 64031, "slurmrestd"])
except subprocess.CalledProcessError as e:
    if e.returncode == 9:
        _logger.debug("group 'slurmrestd' already exists")
    else:
        raise SlurmOpsError(f"failed to create group 'slurmrestd'. reason: {e}")

try:
    subprocess.check_output(
        [
            "adduser",
            "--system",
            "--gid",
            64031,
            "--uid",
            64031,
            "--no-create-home",
            "--home",
            "/nonexistent",
            "slurmrestd",
        ]
    )
except subprocess.CalledProcessError as e:
    if e.returncode == 9:
        _logger.debug("user 'slurmrestd' already exists")
    else:
        raise SlurmOpsError(f"failed to create user 'slurmrestd'. reason: {e}")

_logger.debug("replacing default slurmrestd service file")
override = Path("/usr/lib/systemd/system/slurmrestd.service")
override.write_text(
    textwrap.dedent(
        """
        [Unit]
        Description=Slurm REST daemon
        After=network.target munge.service slurmctld.service
        ConditionPathExists=/etc/slurm/slurm.conf
        Documentation=man:slurmrestd(8)

        [Service]
        Type=simple
        EnvironmentFile=-/etc/default/slurmrestd
        Environment="SLURM_JWT=daemon"
        ExecStart=/usr/sbin/slurmrestd $SLURMRESTD_OPTIONS -vv 0.0.0.0:6820
        ExecReload=/bin/kill -HUP $MAINPID
        User=slurmrestd
        Group=slurmrestd

        # Restart service if failed
        Restart=on-failure
        RestartSec=30s

        [Install]
        WantedBy=multi-user.target
        """
    )
)
_systemctl("daemon-reload")

However, this quick fix should really be upstreamed into the Debian package since not everyone using slurmrestd on Debian/Ubuntu is using the slurm_ops charm library.