Fix the task that acquires hostlist for a controller that has no workers

[afgane]

Otherwise, running this role on a brand new server was reporting:

failed: [jetstream-iu0.galaxyproject.org] (item={u'partition': u'large', u'sockets': 10, u'inventory_group': u'jetstream-iu-large', u'real_memory': 29997}) => {"changed": true, "cmd": ["scontrol", "show", "hostlist"], "delta": "0:00:59.013009", "end": "2016-10-04 19:50:02.470360", "failed": true, "item": {"inventory_group": "jetstream-iu-large", "partition": "large", "real_memory": 29997, "sockets": 10}, "rc": 1, "start": "2016-10-04 19:49:03.457351", "stderr": "scontrol: error: s_p_parse_file: unable to status file /etc/slurm/slurm.conf: No such file or directory, retrying in 1sec up to 60sec\nscontrol: fatal: Unable to process configuration file", "stdout": "", "stdout_lines": [], "warnings": []}

This also fixes the bare variable deprecation warning.