add TaskPlugin to default parameters

These changes address #16 by adding sane defaults for the
TaskPlugin configuration parameter as specified in the
[slurm configuration](https://slurm.schedmd.com/slurm.conf.html#OPT_TaskPlugin).

charms/slurmctld/src/charm.py

@@ -311,6 +311,7 @@ def _assemble_slurmctld_parameters() -> str:
             "SlurmctldHost": self.hostname,
             "SlurmctldParameters": _assemble_slurmctld_parameters(),
             "ProctrackType": "proctrack/linuxproc" if is_container() else "proctrack/cgroup",
+            "TaskPlugin": "task/affinity" if is_container() else "task/cgroup,task/affinity",
             **accounting_params,
             **CHARM_MAINTAINED_SLURM_CONF_PARAMETERS,
             **slurmd_parameters,