Enforce users to specify a 'pool' dimension when triggering a task.

client/example/3_swarming_trigger_collect.py

 #!/usr/bin/env python
 # Copyright 2012 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
 """Runs hello_world.py, through hello_world.isolate, remotely on a Swarming
 slave.
 
 It compiles and archives via 'isolate.py archive', then discard the local files.
 After, it triggers and finally collects the results.
@@ skipping 22 matching lines @@
           tempdir, options.isolate_server, options.swarming_os, options.verbose)
 
       json_file = os.path.join(tempdir, 'task.json')
       common.note('Running on %s' % options.swarming)
       cmd = [
         'swarming.py',
         'trigger',
         '--swarming', options.swarming,
         '--isolate-server', options.isolate_server,
         '--dimension', 'os', options.swarming_os,
+        '--dimension', 'pool', 'default',
         '--task-name', options.task_name,
         '--dump-json', json_file,
         '--isolated', hashval,
         '--shards', '2',
       ]
       if options.idempotent:
         cmd.append('--idempotent')
       if options.priority is not None:
         cmd.extend(('--priority', str(options.priority)))
       cmd.extend(('--', '${ISOLATED_OUTDIR}'))
@@ skipping 16 matching lines @@
             print(f.read())
       return 0
     finally:
       shutil.rmtree(tempdir)
   except subprocess.CalledProcessError as e:
     return e.returncode
 
 
 if __name__ == '__main__':
   sys.exit(main())