Allow arbitrary hostnames

[sjpb]

List nodes and nodes in partitions in slurm.conf using ansible inventory group contents rather than assuming they follow the pattern cluster_name-group_name-{0..num_nodes-1}. This allows compute nodes to have arbitrary hostnames.

I think this addresses #3 in a more flexible way than the current code.

Note that:

• The parameter num_nodes is no longer required & will be ignored if provided.
• The requirement for appropriately-named inventory group(s) existed already; this is not changed by this PR.
• Node definition now makes use of NodeName=DEFAULT which is recommended to improve scheduler performance.
• The case with a single node in a partition/group is now handled without being a special case.
• Combined conditionals on this line avoids a bug in the current master branch with escaped line-ends if ram_mb is not provided and group_hosts is empty.
• Newlines/empty lines in template are required to put linebreaks in completed file in the correct place.

Tested (manually, and now via Molecule running locally with different node/partition/group names - this is not yet in Travis) on cases:

        # test 1: single partition, group is partition
        openhpc_slurm_partitions:
         - name: "compute"
        # test 1b: single node [OK]
        # test 1c: multiple nodes, non-sequential names
        
        # test 2: two partitions, groups are partitions
        openhpc_slurm_partitions:
          - name: "compute"
          - name: "fake"
        
        # test 3: one partition with groups
        openhpc_slurm_partitions:
          - name: "compute"
            groups:
            - name: "compute"
            - name: "fake"

[brtkwr]

FYI the molecule tests pass and also deployed a basic slurm cluster which also appears to work.

[oneswig]

Minor observation on the README content. I tested it with OpenHPC2 and CentOS 8 - there's some work to do there but I ran on top of this branch and it all seemed good.

In due course it would be neat to make a python filter to find run-length encodings from an arbitrary list of hostnames!

[brtkwr]

LGTM