enhancement(slurmctld): enforce job level memory

These changes introduce default values for SelectTypeParameters
and MemSpecLimit.

`SelectTypeParameters=CR_CPU_Memory` to enable memory allocation
enforcement.

`MemSpecLimit=1024` to ensure systems always have 1G available
memory that jobs cannot consume.

charms/slurmctld/src/constants.py

@@ -15,9 +15,11 @@
     "HealthCheckNodeState": ["ANY", "CYCLE"],
     "HealthCheckProgram": "/usr/sbin/charmed-hpc-nhc-wrapper",
     "MailProg": "/usr/bin/mail.mailutils",
+    "MemSpecLimit": "1024",
     "PluginDir": ["/usr/lib/x86_64-linux-gnu/slurm-wlm"],
     "PlugStackConfig": "/etc/slurm/plugstack.conf.d/plugstack.conf",
     "SelectType": "select/cons_tres",
+    "SelectTypeParameters": "CR_CPU_Memory",
     "SlurmctldPort": "6817",
     "SlurmdPort": "6818",
     "StateSaveLocation": "/var/lib/slurm/checkpoint",