Hotfix for clusters from #2701 (#2747)

Fixes an issue created from #2701 that added `CLUSTERS` to the
`gaea.yaml`.

workflow/rocoto/tasks.py

@@ -226,7 +226,7 @@ def get_resource(self, task_name):
             native = '--export=NONE'
             if task_config['RESERVATION'] != "":
                 native += '' if task_name in Tasks.SERVICE_TASKS else ' --reservation=' + task_config['RESERVATION']
-            if task_config['CLUSTERS'] != "":
+            if task_config['CLUSTERS'] not in ["", '@CLUSTERS@']:
                 native += ' --clusters=' + task_config['CLUSTERS']
 
         queue = task_config['QUEUE_SERVICE'] if task_name in Tasks.SERVICE_TASKS else task_config['QUEUE']