Role variables doc improvements

[sjpb]

Some minor issues with the Role Variable docs:

• default missing "this partition is the default".
• Would be good to link variables to their slurm.conf parameters:
  • ram_mb (RealMemory)
  • default (Default)
  • maxtime (MaxTime)
• The README example also shows some additional keys flavor, image and user are not used by the role, which is a bit confusing.
• The link between group/partition names and ansible is shown in the example, but the documentation could be more explicit and it's not stated that it actually links to instance names as well. For each group/partition definition:
  • there must be an ansible group named <cluster_name>_<name> which contains the relevant compute hosts: also note that this means ansible's group name restrictions apply to both bits
  • nodes in this group must have hostnames which are this <cluster_name>_<name>-<num> (where num starts at 0) (NB if slurm.conf incorrect if only one compute node #20 added then the suffix is only required for groups/partitions with >1 node)

[sjpb]

Also see #3 - although it's closed I think it covers some of the same ground.

[sjpb]

openhpc_slurm_service is documented - this doesn't need to be set by the playbook so should it be documented?

Currently:

openhpc_slurm_service_enabled: checks whether openhpc_slurm_service is enabled

think this is misleading - it defines whether it's enabled.